diff --git a/calendar/README.md b/calendar/README.md
index f3fa8c4..f94ace2 100644
--- a/calendar/README.md
+++ b/calendar/README.md
@@ -12,5 +12,5 @@ reliable API documentation please see the relevant [schema files](./schema/).
 | ------------- | --------
 | Description   | Experiment and add-on for calendar-related APIs in Thunderbird.
 | Status        | Draft
-| Compatibility | Thunderbird 91 (possibly Thunderbird 78)
+| Compatibility | Thunderbird 128
 | Tracking      | [bug 1627205](https://bugzilla.mozilla.org/show_bug.cgi?id=1627205)
diff --git a/calendar/experiments/calendar/child/ext-calendar-provider-actor.jsm b/calendar/experiments/calendar/child/ext-calendar-provider-actor.sys.mjs
similarity index 74%
rename from calendar/experiments/calendar/child/ext-calendar-provider-actor.jsm
rename to calendar/experiments/calendar/child/ext-calendar-provider-actor.sys.mjs
index 298fc3f..4322578 100644
--- a/calendar/experiments/calendar/child/ext-calendar-provider-actor.jsm
+++ b/calendar/experiments/calendar/child/ext-calendar-provider-actor.sys.mjs
@@ -2,12 +2,10 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-var EXPORTED_SYMBOLS = ["CalendarProviderChild"];
-
-class CalendarProviderChild extends JSWindowActorChild {
+export class CalendarProviderChild extends JSWindowActorChild {
   receiveMessage(msg) {
     if (msg.name == "postMessage") {
-      this.contentWindow.postMessage(msg.data.message, msg.data.origin)
+      this.contentWindow.postMessage(msg.data.message, msg.data.origin);
     }
   }
 }
diff --git a/calendar/experiments/calendar/ext-calendar-utils.jsm b/calendar/experiments/calendar/ext-calendar-utils.sys.mjs
similarity index 77%
rename from calendar/experiments/calendar/ext-calendar-utils.jsm
rename to calendar/experiments/calendar/ext-calendar-utils.sys.mjs
index 33cf825..08cca19 100644
--- a/calendar/experiments/calendar/ext-calendar-utils.jsm
+++ b/calendar/experiments/calendar/ext-calendar-utils.sys.mjs
@@ -2,52 +2,22 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-var EXPORTED_SYMBOLS = [
-  "isOwnCalendar",
-  "unwrapCalendar",
-  "getResolvedCalendarById",
-  "getCachedCalendar",
-  "isCachedCalendar",
-  "convertCalendar",
-  "propsToItem",
-  "convertItem",
-  "convertAlarm",
-  "setupE10sBrowser",
-];
-
-var { XPCOMUtils } = ChromeUtils.import("resource://gre/modules/XPCOMUtils.jsm");
-
-XPCOMUtils.defineLazyModuleGetters(this, {
-  cal: "resource:///modules/calendar/calUtils.jsm",
-  ICAL: "resource:///modules/calendar/Ical.jsm",
-  CalEvent: "resource:///modules/CalEvent.jsm",
-  CalTodo: "resource:///modules/CalTodo.jsm",
-  ExtensionParent: "resource://gre/modules/ExtensionParent.jsm",
-});
-
-var { ExtensionError, promiseEvent } = ChromeUtils.import(
-  "resource://gre/modules/ExtensionUtils.jsm"
-).ExtensionUtils;
-
-XPCOMUtils.defineLazyGetter(this, "standaloneStylesheets", () => {
-  let stylesheets = [];
-  let { AppConstants } = ChromeUtils.import("resource://gre/modules/AppConstants.jsm");
-
-  if (AppConstants.platform === "macosx") {
-    stylesheets.push("chrome://browser/content/extension-mac-panel.css");
-  } else if (AppConstants.platform === "win") {
-    stylesheets.push("chrome://browser/content/extension-win-panel.css");
-  } else if (AppConstants.platform === "linux") {
-    stylesheets.push("chrome://browser/content/extension-linux-panel.css");
-  }
-  return stylesheets;
-});
+var {
+  ExtensionUtils: { ExtensionError, promiseEvent }
+} = ChromeUtils.importESModule("resource://gre/modules/ExtensionUtils.sys.mjs");
+
+var { cal } = ChromeUtils.importESModule("resource:///modules/calendar/calUtils.sys.mjs");
+var { CalEvent } = ChromeUtils.importESModule("resource:///modules/CalEvent.sys.mjs");
+var { CalTodo } = ChromeUtils.importESModule("resource:///modules/CalTodo.sys.mjs");
+var { ExtensionParent } = ChromeUtils.importESModule("resource://gre/modules/ExtensionParent.sys.mjs");
+
+var { default: ICAL } = ChromeUtils.importESModule("resource:///modules/calendar/Ical.sys.mjs");
 
-function isOwnCalendar(calendar, extension) {
+export function isOwnCalendar(calendar, extension) {
   return calendar.superCalendar.type == "ext-" + extension.id;
 }
 
-function unwrapCalendar(calendar) {
+export function unwrapCalendar(calendar) {
   let unwrapped = calendar.wrappedJSObject;
 
   if (unwrapped.mUncachedCalendar) {
@@ -57,7 +27,7 @@ function unwrapCalendar(calendar) {
   return unwrapped;
 }
 
-function getResolvedCalendarById(extension, id) {
+export function getResolvedCalendarById(extension, id) {
   let calendar;
   if (id.endsWith("#cache")) {
     let cached = cal.manager.getCalendarById(id.substring(0, id.length - 6));
@@ -72,16 +42,16 @@ function getResolvedCalendarById(extension, id) {
   return calendar;
 }
 
-function getCachedCalendar(calendar) {
+export function getCachedCalendar(calendar) {
   return calendar.wrappedJSObject.mCachedCalendar || calendar;
 }
 
-function isCachedCalendar(id) {
+export function isCachedCalendar(id) {
   // TODO make this better
   return id.endsWith("#cache");
 }
 
-function convertCalendar(extension, calendar) {
+export function convertCalendar(extension, calendar) {
   if (!calendar) {
     return null;
   }
@@ -105,7 +75,7 @@ function convertCalendar(extension, calendar) {
   return props;
 }
 
-function propsToItem(props, baseItem) {
+export function propsToItem(props, baseItem) {
   let item;
   if (baseItem) {
     item = baseItem;
@@ -122,7 +92,18 @@ function propsToItem(props, baseItem) {
   if (props.formats?.use == "ical") {
     item.icalString = props.formats.ical;
   } else if (props.formats?.use == "jcal") {
-    item.icalString = ICAL.stringify(props.formats.jcal);
+    try {
+      item.icalString = ICAL.stringify(props.formats.jcal);
+    } catch (e) {
+      let jsonstring;
+      try {
+        jsonstring = JSON.stringify(props.formats.jcal, null, 2);
+      } catch {
+        jsonstring = props.formats.jcal;
+      }
+
+      throw new ExtensionError("Could not parse jCal: " + e + "\n" + jsonstring);
+    }
   } else {
     if (props.id) {
       item.id = props.id;
@@ -155,7 +136,7 @@ function propsToItem(props, baseItem) {
   return item;
 }
 
-function convertItem(item, options, extension) {
+export function convertItem(item, options, extension) {
   if (!item) {
     return null;
   }
@@ -181,7 +162,7 @@ function convertItem(item, options, extension) {
     try {
       // TODO This is a sync operation. Not great. Can we optimize this?
       props.metadata = JSON.parse(cache.getMetaData(item.id)) ?? {};
-    } catch (e) {
+    } catch {
       // Ignore json parse errors
     }
   }
@@ -218,7 +199,7 @@ function convertItem(item, options, extension) {
   return props;
 }
 
-function convertAlarm(item, alarm) {
+export function convertAlarm(item, alarm) {
   const ALARM_RELATED_MAP = {
     [Ci.calIAlarm.ALARM_RELATED_ABSOLUTE]: "absolute",
     [Ci.calIAlarm.ALARM_RELATED_START]: "start",
@@ -234,7 +215,7 @@ function convertAlarm(item, alarm) {
   };
 }
 
-async function setupE10sBrowser(extension, browser, parent, initOptions={}) {
+export async function setupE10sBrowser(extension, browser, parent, initOptions={}) {
   browser.setAttribute("type", "content");
   browser.setAttribute("disableglobalhistory", "true");
   browser.setAttribute("messagemanagergroup", "webext-browsers");
@@ -267,9 +248,10 @@ async function setupE10sBrowser(extension, browser, parent, initOptions={}) {
   let sheets = [];
   if (initOptions.browser_style) {
     delete initOptions.browser_style;
-    sheets.push(...ExtensionParent.extensionStylesheets);
+    sheets.push("chrome://browser/content/extension.css");
   }
-  sheets.push(...standaloneStylesheets);
+  sheets.push("chrome://browser/content/extension-popup-panel.css");
+
 
   const initBrowser = () => {
     ExtensionParent.apiManager.emit("extension-browser-inserted", browser);
diff --git a/calendar/experiments/calendar/parent/ext-calendar-calendars.js b/calendar/experiments/calendar/parent/ext-calendar-calendars.js
index 2d7db92..a39a481 100644
--- a/calendar/experiments/calendar/parent/ext-calendar-calendars.js
+++ b/calendar/experiments/calendar/parent/ext-calendar-calendars.js
@@ -2,14 +2,14 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-var { ExtensionCommon } = ChromeUtils.import("resource://gre/modules/ExtensionCommon.jsm");
-var { ExtensionUtils } = ChromeUtils.import("resource://gre/modules/ExtensionUtils.jsm");
+var {
+  ExtensionCommon: { ExtensionAPI, EventManager }
+} = ChromeUtils.importESModule("resource://gre/modules/ExtensionCommon.sys.mjs");
+var {
+  ExtensionUtils: { ExtensionError }
+} = ChromeUtils.importESModule("resource://gre/modules/ExtensionUtils.sys.mjs");
 
-var { ExtensionAPI, EventManager } = ExtensionCommon;
-var { ExtensionError } = ExtensionUtils;
-
-var { Services } = ChromeUtils.import("resource://gre/modules/Services.jsm");
-var { cal } = ChromeUtils.import("resource:///modules/calendar/calUtils.jsm");
+var { cal } = ChromeUtils.importESModule("resource:///modules/calendar/calUtils.sys.mjs");
 
 this.calendar_calendars = class extends ExtensionAPI {
   getAPI(context) {
@@ -18,7 +18,7 @@ this.calendar_calendars = class extends ExtensionAPI {
       getResolvedCalendarById,
       isOwnCalendar,
       convertCalendar,
-    } = ChromeUtils.import("resource://experiment-calendar/experiments/calendar/ext-calendar-utils.jsm");
+    } = ChromeUtils.importESModule("resource://tb-experiments-calendar/experiments/calendar/ext-calendar-utils.sys.mjs");
 
     return {
       calendar: {
@@ -30,7 +30,7 @@ this.calendar_calendars = class extends ExtensionAPI {
             if (url) {
               try {
                 pattern = new MatchPattern(url, { restrictSchemes: false });
-              } catch (e) {
+              } catch {
                 throw new ExtensionError(`Invalid url pattern: ${url}`);
               }
             }
@@ -127,7 +127,18 @@ this.calendar_calendars = class extends ExtensionAPI {
 
             if (updateProperties.capabilities) {
               // TODO validate capability names
-              calendar.capabilities = Object.assign({}, calendar.capabilities, updateProperties.capabilities);
+              let unwrappedCalendar = calendar.wrappedJSObject.mUncachedCalendar.wrappedJSObject;
+              unwrappedCalendar.capabilities = Object.assign({}, unwrappedCalendar.capabilities, updateProperties.capabilities);
+            }
+
+            if (updateProperties.lastError !== undefined) {
+              if (updateProperties.lastError === null) {
+                calendar.setProperty("currentStatus", Cr.NS_ERROR_FAILURE);
+                calendar.setProperty("lastErrorMessage", updateProperties.lastError);
+              } else {
+                calendar.setProperty("currentStatus", Cr.NS_OK);
+                calendar.setProperty("lastErrorMessage", "");
+              }
             }
           },
           remove: async function(id) {
diff --git a/calendar/experiments/calendar/parent/ext-calendar-items.js b/calendar/experiments/calendar/parent/ext-calendar-items.js
index 4880e59..73e349e 100644
--- a/calendar/experiments/calendar/parent/ext-calendar-items.js
+++ b/calendar/experiments/calendar/parent/ext-calendar-items.js
@@ -2,12 +2,14 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-var { ExtensionCommon } = ChromeUtils.import("resource://gre/modules/ExtensionCommon.jsm");
-var { ExtensionUtils } = ChromeUtils.import("resource://gre/modules/ExtensionUtils.jsm");
-var { cal } = ChromeUtils.import("resource:///modules/calendar/calUtils.jsm");
+var {
+  ExtensionCommon: { ExtensionAPI, EventManager }
+} = ChromeUtils.importESModule("resource://gre/modules/ExtensionCommon.sys.mjs");
+var {
+  ExtensionUtils: { ExtensionError }
+} = ChromeUtils.importESModule("resource://gre/modules/ExtensionUtils.sys.mjs");
 
-var { ExtensionAPI, EventManager } = ExtensionCommon;
-var { ExtensionError } = ExtensionUtils;
+var { cal } = ChromeUtils.importESModule("resource:///modules/calendar/calUtils.sys.mjs");
 
 this.calendar_items = class extends ExtensionAPI {
   getAPI(context) {
@@ -19,13 +21,12 @@ this.calendar_items = class extends ExtensionAPI {
       propsToItem,
       convertItem,
       convertAlarm,
-    } = ChromeUtils.import("resource://experiment-calendar/experiments/calendar/ext-calendar-utils.jsm");
+    } = ChromeUtils.importESModule("resource://tb-experiments-calendar/experiments/calendar/ext-calendar-utils.sys.mjs");
 
     return {
       calendar: {
         items: {
           query: async function(queryProps) {
-            console.log(queryProps);
             let calendars = [];
             if (typeof queryProps.calendarId == "string") {
               calendars = [getResolvedCalendarById(context.extension, queryProps.calendarId)];
@@ -38,9 +39,7 @@ this.calendar_items = class extends ExtensionAPI {
 
             let calendarItems;
             if (queryProps.id) {
-              calendarItems = await Promise.all(calendars.map(calendar => {
-                return calendar.getItem(queryProps.id);
-              }));
+              calendarItems = await Promise.all(calendars.map(calendar => calendar.getItem(queryProps.id)));
             } else {
               calendarItems = await Promise.all(calendars.map(async calendar => {
                 let filter = Ci.calICalendar.ITEM_FILTER_COMPLETED_ALL;
@@ -58,17 +57,16 @@ this.calendar_items = class extends ExtensionAPI {
 
                 let rangeStart = queryProps.rangeStart ? cal.createDateTime(queryProps.rangeStart) : null;
                 let rangeEnd = queryProps.rangeEnd ? cal.createDateTime(queryProps.rangeEnd) : null;
+
                 return calendar.getItemsAsArray(filter, queryProps.limit ?? 0, rangeStart, rangeEnd);
               }));
-              calendarItems = calendarItems.flat();
             }
 
-            return calendarItems.map(item => convertItem(item, queryProps, context.extension));
+            return calendarItems.flat().map(item => convertItem(item, queryProps, context.extension));
           },
           get: async function(calendarId, id, options) {
             let calendar = getResolvedCalendarById(context.extension, calendarId);
             let item = await calendar.getItem(id);
-
             return convertItem(item, options, context.extension);
           },
           create: async function(calendarId, createProperties) {
@@ -139,6 +137,7 @@ this.calendar_items = class extends ExtensionAPI {
           },
           remove: async function(calendarId, id) {
             let calendar = getResolvedCalendarById(context.extension, calendarId);
+
             let item = await calendar.getItem(id);
             if (!item) {
               throw new ExtensionError("Could not find item " + id);
diff --git a/calendar/experiments/calendar/parent/ext-calendar-provider.js b/calendar/experiments/calendar/parent/ext-calendar-provider.js
index 5fb158f..550dbfc 100644
--- a/calendar/experiments/calendar/parent/ext-calendar-provider.js
+++ b/calendar/experiments/calendar/parent/ext-calendar-provider.js
@@ -2,17 +2,35 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-var { ExtensionCommon } = ChromeUtils.import("resource://gre/modules/ExtensionCommon.jsm");
-var { Services } = ChromeUtils.import("resource://gre/modules/Services.jsm");
-var { cal } = ChromeUtils.import("resource:///modules/calendar/calUtils.jsm");
+var {
+  ExtensionCommon: { ExtensionAPI, EventManager }
+} = ChromeUtils.importESModule("resource://gre/modules/ExtensionCommon.sys.mjs");
 
-var { ExtensionAPI, EventManager } = ExtensionCommon;
+var { cal } = ChromeUtils.importESModule("resource:///modules/calendar/calUtils.sys.mjs");
+var { ExtensionSupport } = ChromeUtils.importESModule("resource:///modules/ExtensionSupport.sys.mjs");
 
-ChromeUtils.defineModuleGetter(
-  this,
-  "ExtensionSupport",
-  "resource:///modules/ExtensionSupport.jsm",
-);
+// TODO move me
+class ItemError extends Error {
+  static CONFLICT = "CONFLICT";
+  static READ_FAILED = "READ_FAILED";
+  static MODIFY_FAILED = "MODIFY_FAILED";
+
+  constructor(reason) {
+    super();
+    this.reason = reason;
+  }
+
+  get xpcomReason() {
+    switch (this.reason) {
+      case ItemError.READ_FAILED:
+        return Ci.calIErrors.READ_FAILED;
+      case ItemError.MODIFY_FAILED:
+        return Ci.calIErrors.MODIFICATION_FAILED;
+      default:
+        return Cr.NS_ERROR_FAILURE;
+    }
+  }
+}
 
 function convertProps(props, extension) {
   let calendar = new ExtCalendar(extension);
@@ -30,10 +48,9 @@ class ExtCalendarProvider {
   QueryInterface = ChromeUtils.generateQI(["calICalendarProvider"]);
 
   static register(extension) {
-    let calmgr = cal.manager;
     let type = "ext-" + extension.id;
 
-    calmgr.registerCalendarProvider(
+    cal.manager.registerCalendarProvider(
       type,
       class extends ExtCalendar {
         constructor() {
@@ -47,15 +64,11 @@ class ExtCalendarProvider {
   }
 
   static unregister(extension) {
-    let calmgr = cal.manager;
     let type = "ext-" + extension.id;
-    calmgr.unregisterCalendarProvider(type, true);
+    cal.manager.unregisterCalendarProvider(type, true);
     cal.provider.unregister(type);
   }
 
-  _cachedAdoptItemCallback = null;
-  _cachedModifyItemCallback = null;
-
   constructor(extension) {
     this.extension = extension;
   }
@@ -70,10 +83,10 @@ class ExtCalendarProvider {
   }
 
   createCalendar() {
-    throw new Error("Not implemented", Cr.NS_ERROR_NOT_IMPLEMENTED);
+    throw new Components.Exception("Not implemented", Cr.NS_ERROR_NOT_IMPLEMENTED);
   }
   deleteCalendar() {
-    throw new Error("Not implemented", Cr.NS_ERROR_NOT_IMPLEMENTED);
+    throw new Components.Exception("Not implemented", Cr.NS_ERROR_NOT_IMPLEMENTED);
   }
 
   getCalendar(url) {
@@ -161,7 +174,6 @@ class ExtCalendar extends cal.provider.BaseClass {
         }
         break;
 
-
       case "readOnly":
         if (this.capabilities.mutable === false) {
           return true;
@@ -208,10 +220,13 @@ class ExtCalendar extends cal.provider.BaseClass {
     return super.getProperty(name);
   }
 
-  async addItem(aItem) {
+  _cachedAdoptItemCallback = null;
+
+  addItem(aItem) {
     return this.adoptItem(aItem.clone());
   }
   async adoptItem(aItem) {
+    const adoptCallback = this._cachedAdoptItemCallback;
     try {
       let items = await this.extension.emit("calendar.provider.onItemCreated", this, aItem);
       let { item, metadata } = items.find(props => props.item) || {};
@@ -227,46 +242,68 @@ class ExtCalendar extends cal.provider.BaseClass {
         // The ID of the item has changed. We'll have to make sure that whatever old item is in the
         // cache is removed.
         // TODO Test this well or risk data loss
-        await this.deleteItem(aItem);
+        await this.offlineStorage.deleteItem(aItem);
       }
 
-      if (!item.calendar) {
-        item.calendar = this.superCalendar;
-      }
+      item.calendar = this.superCalendar;
+
       this.observers.notify("onAddItem", [item]);
-      if (this._cachedAdoptItemCallback) {
-        await this._cachedAdoptItemCallback(
-          this.superCalendar,
-          Cr.NS_OK,
-          Ci.calIOperationListener.ADD,
-          item.id,
-          item
-        );
+
+      if (adoptCallback) {
+        await adoptCallback(item.calendar, Cr.NS_OK, Ci.calIOperationListener.ADD, item.id, item);
       }
       return item;
     } catch (e) {
-      if (this._cachedAdoptItemCallback) {
-        await this._cachedAdoptItemCallback(
-          this.superCalendar,
-          e.result || Cr.NS_ERROR_FAILURE,
-          Ci.calIOperationListener.ADD,
-          aItem.id,
-          aItem
-        );
+      let code;
+      if (e instanceof ItemError) {
+        code = e.xpcomReason;
+      } else {
+        code = e.result || Cr.NS_ERROR_FAILURE;
+      }
+
+      throw new Components.Exception(e.message, code);
+    }
+  }
+
+  discoverItem(results) {
+    let error, success;
+
+    for (let result of results) {
+      if (typeof result == "object" && result?.error) {
+        success = null;
+        error = result.error;
+        break;
+      }
+
+      if (typeof result == "object" && result?.item) {
+        success = result;
       }
-      throw e;
+      // TODO warn if two results?
+    }
+
+    if (error) {
+      throw new ItemError(error);
+    } else {
+      return success;
     }
   }
 
-  async modifyItem(aNewItem, aOldItem) {
+  _cachedModifyItemCallback = null;
+
+  async modifyItem(aNewItem, aOldItem, aOptions = {}) {
+    const modifyCallback = this._cachedModifyItemCallback;
+
     try {
-      let items = await this.extension.emit(
+      let results = await this.extension.emit(
         "calendar.provider.onItemUpdated",
         this,
         aNewItem,
-        aOldItem
+        aOldItem,
+        aOptions
       );
-      let { item, metadata } = items.find(props => props.item) || {};
+
+      let { item, metadata } = this.discoverItem(results);
+
       if (!item) {
         throw new Components.Exception("Did not receive item from extension", Cr.NS_ERROR_FAILURE);
       }
@@ -279,40 +316,72 @@ class ExtCalendar extends cal.provider.BaseClass {
         item.calendar = this.superCalendar;
       }
       this.observers.notify("onModifyItem", [item, aOldItem]);
-      if (this._cachedModifyItemCallback) {
-        await this._cachedModifyItemCallback(
-          this.superCalendar,
-          Cr.NS_OK,
-          Ci.calIOperationListener.MODIFY,
-          item.id,
-          item
-        );
+      if (modifyCallback) {
+        await modifyCallback(item.calendar, Cr.NS_OK, Ci.calIOperationListener.MODIFY, item.id, item);
       }
       return item;
     } catch (e) {
-      if (this._cachedModifyItemCallback) {
-        await this._cachedModifyItemCallback(
-          this.superCalendar,
-          e.result || Cr.NS_ERROR_FAILURE,
-          Ci.calIOperationListener.MODIFY,
-          aNewItem.id,
-          aNewItem
-        );
+      let code;
+      if (e instanceof ItemError) {
+        if (e.reason == ItemError.CONFLICT) {
+          let overwrite = cal.provider.promptOverwrite("modify", aOldItem);
+          if (overwrite) {
+            return this.modifyItem(aNewItem, aOldItem, { force: true });
+          } else {
+            code = Ci.calIErrors.OPERATION_CANCELLED;
+            this.superCalendar.refresh();
+          }
+        } else {
+          code = e.xpcomReason;
+        }
+      } else {
+        code = e.result || Cr.NS_ERROR_FAILURE;
       }
-      throw e;
+      throw new Components.Exception(e.message, code);
     }
   }
 
-  async deleteItem(aItem, aListener) {
-    let results = await this.extension.emit("calendar.provider.onItemRemoved", this, aItem);
-    if (!results.length) {
-      throw new Components.Exception(
-        "Extension did not consume item deletion",
-        Cr.NS_ERROR_FAILURE
+  async deleteItem(aItem, aOptions = {}) {
+    try {
+      let results = await this.extension.emit(
+        "calendar.provider.onItemRemoved",
+        this,
+        aItem,
+        aOptions
       );
-    }
 
-    this.observers.notify("onDeleteItem", [aItem]);
+      if (!results.length) {
+        throw new Components.Exception(
+          "Extension did not consume item deletion",
+          Cr.NS_ERROR_FAILURE
+        );
+      }
+
+      // This will discover errors and throw them
+      this.discoverItem(results);
+
+      this.observers.notify("onDeleteItem", [aItem]);
+    } catch (e) {
+      let code;
+      if (e instanceof ItemError) {
+        if (e.reason == ItemError.CONFLICT) {
+          let overwrite = cal.provider.promptOverwrite("delete", aItem);
+          if (overwrite) {
+            return this.deleteItem(aItem, { force: true });
+          } else {
+            code = Ci.calIErrors.OPERATION_CANCELLED;
+            this.superCalendar.refresh();
+          }
+        } else {
+          code = e.xpcomReason;
+        }
+      } else {
+        code = e.result || Cr.NS_ERROR_FAILURE;
+      }
+
+      throw new Components.Exception(e.message, code);
+    }
+    return aItem;
   }
 
   getItem(aId) {
@@ -386,15 +455,14 @@ this.calendar_provider = class extends ExtensionAPI {
       this.onManifestEntry("calendar_provider");
     }
 
-    // TODO Change this for your add-on to avoid conflicts
     Services.io
       .getProtocolHandler("resource")
       .QueryInterface(Ci.nsIResProtocolHandler)
-      .setSubstitution("experiment-calendar", this.extension.rootURI);
+      .setSubstitution("tb-experiments-calendar", this.extension.rootURI);
 
-    const { setupE10sBrowser } = ChromeUtils.import("resource://experiment-calendar/experiments/calendar/ext-calendar-utils.jsm");
+    const { setupE10sBrowser } = ChromeUtils.importESModule("resource://tb-experiments-calendar/experiments/calendar/ext-calendar-utils.sys.mjs");
 
-    ChromeUtils.registerWindowActor("CalendarProvider", { child: { moduleURI: "resource://experiment-calendar/experiments/calendar/child/ext-calendar-provider-actor.jsm" } });
+    ChromeUtils.registerWindowActor("CalendarProvider", { child: { esModuleURI: "resource://tb-experiments-calendar/experiments/calendar/child/ext-calendar-provider-actor.sys.mjs" } });
 
     ExtensionSupport.registerWindowListener("ext-calendar-provider-" + this.extension.id, {
       chromeURLs: ["chrome://calendar/content/calendar-creation.xhtml"],
@@ -416,7 +484,9 @@ this.calendar_provider = class extends ExtensionAPI {
             }
 
             loadPromise.then(() => {
-              browser.loadURI(calendarType.panelSrc, { triggeringPrincipal: this.extension.principal });
+              browser.fixupAndLoadURIString(calendarType.panelSrc, {
+                triggeringPrincipal: this.extension.principal
+              });
             });
 
             win.gButtonHandlers.forNodeId["panel-addon-calendar-settings"].accept = calendarType.onCreated;
@@ -447,13 +517,11 @@ this.calendar_provider = class extends ExtensionAPI {
       ExtCalendarProvider.unregister(this.extension);
     }
 
-    Cu.unload("resource://experiment-calendar/experiments/calendar/ext-calendar-utils.jsm");
-
+    Cu.unload("resource://tb-experiments-calendar/experiments/calendar/ext-calendar-utils.sys.mjs");
     Services.io
       .getProtocolHandler("resource")
       .QueryInterface(Ci.nsIResProtocolHandler)
-      .setSubstitution("experiment-calendar", null);
-
+      .setSubstitution("tb-experiments-calendar", null);
     Services.obs.notifyObservers(null, "startupcache-invalidate", null);
   }
 
@@ -485,7 +553,7 @@ this.calendar_provider = class extends ExtensionAPI {
       propsToItem,
       convertItem,
       convertCalendar,
-    } = ChromeUtils.import("resource://experiment-calendar/experiments/calendar/ext-calendar-utils.jsm");
+    } = ChromeUtils.importESModule("resource://tb-experiments-calendar/experiments/calendar/ext-calendar-utils.sys.mjs");
 
     return {
       calendar: {
@@ -494,18 +562,24 @@ this.calendar_provider = class extends ExtensionAPI {
             context,
             name: "calendar.provider.onItemCreated",
             register: (fire, options) => {
-              let listener = async (event, calendar, item) => {
+              let listener = async (event, calendar, item, listenerOptions) => {
                 let props = await fire.async(
                   convertCalendar(context.extension, calendar),
-                  convertItem(item, options, context.extension)
+                  convertItem(item, options, context.extension),
+                  listenerOptions
                 );
-                if (props?.type) {
-                  item = propsToItem(props, item);
-                }
-                if (!item.id) {
-                  item.id = cal.getUUID();
+
+                if (props?.error) {
+                  return { error: props.error };
+                } else {
+                  if (props?.type) {
+                    item = propsToItem(props, item);
+                  }
+                  if (!item.id) {
+                    item.id = cal.getUUID();
+                  }
+                  return { item, metadata: props?.metadata };
                 }
-                return { item, metadata: props?.metadata };
               };
 
               context.extension.on("calendar.provider.onItemCreated", listener);
@@ -519,16 +593,21 @@ this.calendar_provider = class extends ExtensionAPI {
             context,
             name: "calendar.provider.onItemUpdated",
             register: (fire, options) => {
-              let listener = async (event, calendar, item, oldItem) => {
+              let listener = async (event, calendar, item, oldItem, listenerOptions) => {
                 let props = await fire.async(
                   convertCalendar(context.extension, calendar),
                   convertItem(item, options, context.extension),
-                  convertItem(oldItem, options, context.extension)
+                  convertItem(oldItem, options, context.extension),
+                  listenerOptions
                 );
-                if (props?.type) {
-                  item = propsToItem(props, item);
+                if (props?.error) {
+                  return { error: props.error };
+                } else {
+                  if (props?.type) {
+                    item = propsToItem(props, item);
+                  }
+                  return { item, metadata: props?.metadata };
                 }
-                return { item, metadata: props?.metadata };
               };
 
               context.extension.on("calendar.provider.onItemUpdated", listener);
@@ -542,11 +621,13 @@ this.calendar_provider = class extends ExtensionAPI {
             context,
             name: "calendar.provider.onItemRemoved",
             register: (fire, options) => {
-              let listener = (event, calendar, item) => {
-                return fire.async(
+              let listener = async (event, calendar, item, listenerOptions) => {
+                let res = await fire.async(
                   convertCalendar(context.extension, calendar),
-                  convertItem(item, options, context.extension)
+                  convertItem(item, options, context.extension),
+                  listenerOptions
                 );
+                return res;
               };
 
               context.extension.on("calendar.provider.onItemRemoved", listener);
diff --git a/calendar/experiments/calendar/parent/ext-calendarItemAction.js b/calendar/experiments/calendar/parent/ext-calendarItemAction.js
index c2acc58..28ccec4 100644
--- a/calendar/experiments/calendar/parent/ext-calendarItemAction.js
+++ b/calendar/experiments/calendar/parent/ext-calendarItemAction.js
@@ -2,29 +2,11 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-ChromeUtils.defineModuleGetter(
-  this,
-  "ToolbarButtonAPI",
-  "resource:///modules/ExtensionToolbarButtons.jsm"
-);
-ChromeUtils.defineModuleGetter(
-  this,
-  "ExtensionParent",
-  "resource://gre/modules/ExtensionParent.jsm"
-);
-
-ChromeUtils.defineModuleGetter(
-  this,
-  "ExtensionSupport",
-  "resource:///modules/ExtensionSupport.jsm",
-);
-
-
-var { ExtensionCommon } = ChromeUtils.import(
-  "resource://gre/modules/ExtensionCommon.jsm"
-);
-
-var { makeWidgetId } = ExtensionCommon;
+var { ExtensionCommon: { makeWidgetId } } = ChromeUtils.importESModule("resource://gre/modules/ExtensionCommon.sys.mjs");
+
+var { ExtensionParent } = ChromeUtils.importESModule("resource://gre/modules/ExtensionParent.sys.mjs");
+var { ExtensionSupport } = ChromeUtils.importESModule("resource:///modules/ExtensionSupport.sys.mjs");
+var { ToolbarButtonAPI } = ChromeUtils.importESModule("resource:///modules/ExtensionToolbarButtons.sys.mjs");
 
 const calendarItemActionMap = new WeakMap();
 
diff --git a/calendar/experiments/calendar/parent/ext-calendarItemDetails.js b/calendar/experiments/calendar/parent/ext-calendarItemDetails.js
index c87e25f..f2e039a 100644
--- a/calendar/experiments/calendar/parent/ext-calendarItemDetails.js
+++ b/calendar/experiments/calendar/parent/ext-calendarItemDetails.js
@@ -2,115 +2,27 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-ChromeUtils.defineModuleGetter(
-  this,
-  "ExtensionSupport",
-  "resource:///modules/ExtensionSupport.jsm",
-);
-
-var { XPCOMUtils } = ChromeUtils.import("resource://gre/modules/XPCOMUtils.jsm");
-var { ExtensionCommon } = ChromeUtils.import("resource://gre/modules/ExtensionCommon.jsm");
-var { ExtensionUtils } = ChromeUtils.import("resource://gre/modules/ExtensionUtils.jsm");
-
-var { promiseEvent } = ExtensionUtils;
-var { makeWidgetId, ExtensionAPI } = ExtensionCommon;
-
-
-XPCOMUtils.defineLazyGetter(this, "standaloneStylesheets", () => {
-  let stylesheets = [];
-  let { AppConstants } = ChromeUtils.import("resource://gre/modules/AppConstants.jsm");
-
-  if (AppConstants.platform === "macosx") {
-    stylesheets.push("chrome://browser/content/extension-mac-panel.css");
-  } else if (AppConstants.platform === "win") {
-    stylesheets.push("chrome://browser/content/extension-win-panel.css");
-  } else if (AppConstants.platform === "linux") {
-    stylesheets.push("chrome://browser/content/extension-linux-panel.css");
-  }
-  return stylesheets;
-});
+var {
+  ExtensionCommon: { ExtensionAPI, makeWidgetId }
+} = ChromeUtils.importESModule("resource://gre/modules/ExtensionCommon.sys.mjs");
 
+var { ExtensionSupport } = ChromeUtils.importESModule("resource:///modules/ExtensionSupport.sys.mjs");
 
 this.calendarItemDetails = class extends ExtensionAPI {
-  async _attachBrowser(tabpanel) {
-    let document = tabpanel.ownerDocument;
-    let browser = document.createXULElement("browser");
-    browser.setAttribute("flex", "1");
-    browser.setAttribute("type", "content");
-    browser.setAttribute("disableglobalhistory", "true");
-    browser.setAttribute("messagemanagergroup", "webext-browsers");
-    browser.setAttribute("transparent", "true");
-    browser.setAttribute("class", "webextension-popup-browser");
-    browser.setAttribute("webextension-view-type", "subview");
-
-    // Ensure the browser will initially load in the same group as other browsers from the same
-    // extension.
-    browser.setAttribute(
-      "initialBrowsingContextGroupId",
-      this.extension.policy.browsingContextGroupId
-    );
-
-    if (this.extension.remote) {
-      browser.setAttribute("remote", "true");
-      browser.setAttribute("remoteType", this.extension.remoteType);
-      browser.setAttribute("maychangeremoteness", "true");
-    }
-
-    let readyPromise;
-    if (this.extension.remote) {
-      readyPromise = promiseEvent(browser, "XULFrameLoaderCreated");
-    } else {
-      readyPromise = promiseEvent(browser, "load");
-    }
-
-    tabpanel.appendChild(browser);
-
-    if (!this.extension.remote) {
-      // FIXME: bug 1494029 - this code used to rely on the browser binding
-      // accessing browser.contentWindow. This is a stopgap to continue doing
-      // that, but we should get rid of it in the long term.
-      browser.contentwindow; // eslint-disable-line no-unused-expressions
-    }
-
-    let sheets = [];
-    if (this.extension.manifest.calendar_item_details.browser_style) {
-      sheets.push(...ExtensionParent.extensionStylesheets);
-    }
-    sheets.push(...standaloneStylesheets);
-
-
-    const initBrowser = () => {
-      ExtensionParent.apiManager.emit("extension-browser-inserted", browser);
-      let mm = browser.messageManager;
-      mm.loadFrameScript(
-        "chrome://extensions/content/ext-browser-content.js",
-        false,
-        true
-      );
-
-      mm.sendAsyncMessage("Extension:InitBrowser", {
-        allowScriptsToClose: true,
-        blockParser: false,
-        maxWidth: 800,
-        maxHeight: 600,
-        stylesheets: sheets
-      });
-    };
-    browser.addEventListener("DidChangeBrowserRemoteness", initBrowser);
-
-    return readyPromise.then(() => {
-      initBrowser();
-      browser.loadURI(this.extension.manifest.calendar_item_details.default_content, { triggeringPrincipal: this.extension.principal });
-    });
-  }
-
   onLoadCalendarItemPanel(window, origLoadCalendarItemPanel, iframeId, url) {
+    const { setupE10sBrowser } = ChromeUtils.importESModule("resource://tb-experiments-calendar/experiments/calendar/ext-calendar-utils.sys.mjs");
+
     let res = origLoadCalendarItemPanel(iframeId, url);
     if (this.extension.manifest.calendar_item_details) {
-      let panelFrame = window.document.getElementById(iframeId || "calendar-item-panel-iframe");
+      let panelFrame;
+      if (window.tabmail) {
+        panelFrame = window.document.getElementById(iframeId|| tabmail.currentTabInfo.iframe?.id);
+      } else {
+        panelFrame = window.document.getElementById("calendar-item-panel-iframe");
+      }
+
       panelFrame.contentWindow.addEventListener("load", (event) => {
         let document = event.target.ownerGlobal.document;
-        console.log(this.extension.manifest.calendar_item_details);
 
         let widgetId = makeWidgetId(this.extension.id);
 
@@ -128,7 +40,15 @@ this.calendarItemDetails = class extends ExtensionAPI {
         tabpanel.setAttribute("id", widgetId + "-calendarItemDetails-tabpanel");
         tabpanel.setAttribute("flex", "1");
 
-        this._attachBrowser(tabpanel);
+        let browser = document.createXULElement("browser");
+        browser.setAttribute("flex", "1");
+        let loadPromise = setupE10sBrowser(this.extension, browser, tabpanel);
+
+        return loadPromise.then(() => {
+          browser.fixupAndLoadURIString(this.extension.manifest.calendar_item_details.default_content, {
+            triggeringPrincipal: this.extension.principal
+          });
+        });
       });
     }
 
@@ -158,13 +78,29 @@ this.calendarItemDetails = class extends ExtensionAPI {
         "chrome://calendar/content/calendar-event-dialog.xhtml"
       ],
       onLoadWindow: (window) => {
-        let orig = window.onLoadCalendarItemPanel;
-        window.onLoadCalendarItemPanel = this.onLoadCalendarItemPanel.bind(this, window, orig.bind(window));
+        if (window.location.href == "chrome://messenger/content/messenger.xhtml") {
+          let orig = window.onLoadCalendarItemPanel;
+          window.onLoadCalendarItemPanel = this.onLoadCalendarItemPanel.bind(this, window, orig.bind(window));
+          window._onLoadCalendarItemPanelOrig = orig;
+        } else {
+          window.setTimeout(() => {
+            this.onLoadCalendarItemPanel(window, () => {});
+          }, 0);
+        }
       }
     });
   }
   onShutdown() {
     ExtensionSupport.unregisterWindowListener("ext-calendarItemDetails-" + this.extension.id);
+
+    for (let wnd of ExtensionSupport.openWindows) {
+      if (wnd.location.href == "chrome://messenger/content/messenger.xhtml") {
+        if (wnd._onLoadCalendarItemPanelOrig) {
+          wnd.onLoadCalendarItemPanel = wnd._onLoadCalendarItemPanelOrig;
+          wnd._onLoadCalendarItemPanelOrig = null;
+        }
+      }
+    }
   }
   getAPI(context) {
     return { calendar: { itemDetails: {} } };
diff --git a/calendar/experiments/calendar/schema/calendar-calendars.json b/calendar/experiments/calendar/schema/calendar-calendars.json
index 28bd10e..7b81444 100644
--- a/calendar/experiments/calendar/schema/calendar-calendars.json
+++ b/calendar/experiments/calendar/schema/calendar-calendars.json
@@ -151,7 +151,8 @@
               "readOnly": { "type": "boolean", "optional": true },
               "enabled": { "type": "boolean", "optional": true },
               "color": { "type": "string", "optional": true },
-              "capabilities": { "$ref": "CalendarCapabilities", "optional": true }
+              "capabilities": { "$ref": "CalendarCapabilities", "optional": true },
+              "lastError": { "type": "string", "optional": true }
             }
           }
         ]
diff --git a/calendar/experiments/calendar/schema/calendar-provider.json b/calendar/experiments/calendar/schema/calendar-provider.json
index 3ff305a..36abeab 100644
--- a/calendar/experiments/calendar/schema/calendar-provider.json
+++ b/calendar/experiments/calendar/schema/calendar-provider.json
@@ -22,13 +22,41 @@
   },
   {
     "namespace": "calendar.provider",
+    "types": [{
+      "id": "ItemError",
+      "type": "object",
+      "description": "A failure in an onItem* handler",
+      "properties": {
+        "error": {
+          "type": "string",
+          "enum": [
+            "GENERAL_FAILURE",
+            "READ_FAILED",
+            "MODIFY_FAILED",
+            "CONFLICT"
+          ]
+        }
+      }
+    }, {
+      "id": "ItemOptions",
+      "type": "object",
+      "description": "Options for the create/modify/delete event handlers",
+      "properties": {
+        "force": {
+          "type": "boolean",
+          "description": "If true, instruct the provider to force overwrite changes (i.e. after a conflict)",
+          "optional": true
+        }
+      }
+    }],
     "events": [
       {
         "name": "onItemCreated",
         "type": "function",
         "parameters": [
           { "name": "calendar", "$ref": "calendar.calendars.Calendar" },
-          { "name": "item", "$ref": "calendar.items.CalendarItem" }
+          { "name": "item", "$ref": "calendar.items.CalendarItem" },
+          { "name": "options", "$ref": "calendar.provider.ItemOptions" }
         ],
         "extraParameters": [
           {
@@ -38,7 +66,14 @@
               "returnFormat": { "$ref": "calendar.items.ReturnFormat", "optional": true }
             }
           }
-        ]
+        ],
+        "returns": {
+          "description": "Returns the added item, or an error that occurred",
+          "choices": [
+            { "$ref": "calendar.provider.ItemError" },
+            { "$ref": "calendar.items.CalendarItem" }
+          ]
+        }
       },
       {
         "name": "onItemUpdated",
@@ -46,7 +81,8 @@
         "parameters": [
           { "name": "calendar", "$ref": "calendar.calendars.Calendar" },
           { "name": "item", "$ref": "calendar.items.CalendarItem" },
-          { "name": "oldItem", "$ref": "calendar.items.CalendarItem" }
+          { "name": "oldItem", "$ref": "calendar.items.CalendarItem" },
+          { "name": "options", "$ref": "calendar.provider.ItemOptions" }
         ],
         "extraParameters": [
           {
@@ -56,14 +92,22 @@
               "returnFormat": { "$ref": "calendar.items.ReturnFormat", "optional": true }
             }
           }
-        ]
+        ],
+        "returns": {
+          "description": "Returns the modified item, or an error that occurred",
+          "choices": [
+            { "$ref": "calendar.provider.ItemError" },
+            { "$ref": "calendar.items.CalendarItem" }
+          ]
+        }
       },
       {
         "name": "onItemRemoved",
         "type": "function",
         "parameters": [
           { "name": "calendar", "$ref": "calendar.calendars.Calendar" },
-          { "name": "item", "$ref": "calendar.items.CalendarItem" }
+          { "name": "item", "$ref": "calendar.items.CalendarItem" },
+          { "name": "options", "$ref": "calendar.provider.ItemOptions" }
         ],
         "extraParameters": [
           {
@@ -73,7 +117,12 @@
               "returnFormat": { "$ref": "calendar.items.ReturnFormat", "optional": true }
             }
           }
-        ]
+        ],
+        "returns": {
+          "description": "Optionally returns an item error if it occurred",
+          "optional": true,
+          "$ref": "calendar.provider.ItemError"
+        }
       },
       {
         "name": "onInit",
diff --git a/calendar/experiments/calendar/schema/calendarItemAction.json b/calendar/experiments/calendar/schema/calendarItemAction.json
index 2288b0a..093e384 100644
--- a/calendar/experiments/calendar/schema/calendarItemAction.json
+++ b/calendar/experiments/calendar/schema/calendarItemAction.json
@@ -50,6 +50,13 @@
                 "description": "Currently unused.",
                 "type": "string",
                 "optional": true
+              },
+              "type": {
+                "description": "Specifies the type of the button. Default type is <var>button</var>.",
+                "type": "string",
+                "enum": ["button", "menu"],
+                "optional": true,
+                "default": "button"
               }
             },
             "optional": true
diff --git a/calendar/manifest.json b/calendar/manifest.json
index 513362e..d1c458e 100644
--- a/calendar/manifest.json
+++ b/calendar/manifest.json
@@ -4,7 +4,7 @@
   "author": "Philipp Kewisch",
   "description": "Draft experiment for Thunderbird MailExtensions covering calendaring",
   "version": "2.2.0",
-  "applications": {
+  "browser_specific_settings": {
     "gecko": {
       "id": "ext-calendar-draft@mozilla.kewis.ch"
     }
@@ -44,7 +44,8 @@
   "calendar_item_action": {
     "default_icon": "addon.png",
     "default_popup": "content/calendar-item-action.html",
-    "default_title": "Calendar Item Action"
+    "default_title": "Calendar Item Action",
+    "type": "button"
   },
 
   "background": {