Add-on for Thunderbird which allows the replied and forwarded flags on existing messages.
source code | addons.thunderbird.net home page
This add-on adds a second "Mark" menu near the bottom of the context menu for the message list. It's a separate menu because Thunderbird does not currently allow add-ons to add menu items to submenus created by Thunderbird itself.
On this submenu are two items, "As Replied" and "As Forwarded". If the message you right-clicked on to bring up the menu was already marked replied and/or forwarded, one or both of the menu items will be checked. Selecting one of the menu items will either mark or unmark the message as appropriate, i.e., it will reverse whatever the state of the checkbox is.
Beware that if you select more than one message before clicking, the add-on looks at only the first of them (where "first" is defined arbitrarily by Thunderbird) to figure out whether to mark or unmark messages.
Development and maintenance of this add-on to keep it free and open-source is supported by Extended Thunder Inc. You may make a recurring or one-time donation to Extended Thunder. Your donation is tax-deductible in the United States as permitted by law.
This version of this add-on is maintained by Jonathan Kamens [email]. You may email bug reports to him, or post them on Github.
Thank you to Christian Eyrich, the original author of this add-on. This version has been almost entirely rewritten to support new Thunderbird versions, but his original add-on served as inspiration and pointed us in the right direction for how to leverage Thunderbird's innards to make this possible.