Add check for manifest.json changes before continuing workflow

.github/workflows/firefox-addon.yml

@@ -13,6 +13,22 @@ jobs:
       - name: Checkout code
         uses: actions/checkout@v2
 
+      - name: Check if manifest.json has changed
+        id: check-manifest
+        run: |
+          CHANGED_FILES=$(git diff --name-only HEAD^ HEAD)
+          if ! echo "$CHANGED_FILES" | grep -q "manifest.json"; then
+            echo "manifest.json has not changed, exiting gracefully"
+            echo "::set-output name=manifest_changed::false"
+          else
+            echo "manifest.json has changed, continuing with the workflow"
+            echo "::set-output name=manifest_changed::true"
+          fi
+          
+      - name: Exit if manifest.json hasn't changed
+        if: ${{ steps.check-manifest.outputs.manifest_changed == 'false' }}
+        run: exit 0
+
       - name: Setup Node.js
         uses: actions/setup-node@v2
         with: