Unable to install add-ons (extensions or themes) in Thunderbird

Revision Information
  • Revision id: 169212
  • Created:
  • Creator: Wayne Mery
  • Comment: initial draft - needs much more change
  • Reviewed: No
  • Ready for localization: No
Revision Source
Revision Content
In Firefox version 57 and above, only extensions built using WebExtensions APIs will work. See Firefox add-on technology is modernizing and these Frequently Asked Questions for details.

An inability to install an extension can be the result of problems with the extension, or the version of Thunderbird you are using. This article describes various causes of install problems and what you can do to address them.

Add-on is not compatible

An internal file in each add-on indicates the range of Firefox versions with which the extension is compatible. In the Thunderbird Add-ons area of Mozilla, you can review version compatibility information on the page for each available extension.

If your installed version of Thunderbird does not fit within that range, installing the add-on results in an error, indicating that the add-on is not compatible. If your version of Firefox is incompatible with the add-on, you can:

  • Update to the latest version of Thunderbird. For more information, see Update Firefox to the latest release.
  • Find a version of the add-on that is compatible with your version of Firefox. The Thunderbird Add-ons website usually has the most up to date version of each add-on.
  • Use a different extension. For many extensions, there are similar extensions that provide the same functionality. For more information, see Thunderbird Add-ons.

Add-on has a high risk of causing stability or security problems and can't be installed

Certain add-ons have been blocklisted for being unsafe. For more information, see Why does Mozilla disable some add-ons from running in Firefox?.

Thunderbird prevented the site from asking you to install software

By default, Thunderbird permits you to install add-ons only from trusted websites. If the website from which you are installing add-on is not on the trusted websites list, a yellow bar appears across the top of the web page with a message similar to the following:

Thunderbird prevented this site (www.example.com) from asking you to install software on your computer.

To allow the site, click Allow in the yellow bar.

To add the site to the trusted sites list:

  1. In the Menu bar at the top of the screen, click Firefox and select Settings (select Preferences on older macOS versions).Click the menu button Fx89menuButton and select Settings.
  2. Select the Security panelPrivacy & Security panel and go down to the Permissions section.
  3. At the right of Warn me when sites try to install add-ons,Warn you when sites try to install add-ons, click Exceptions….
  4. In the Allowed Sites - Add-ons InstallationAllowed Websites - Add-ons Installation dialog, the list of permitted sites is displayed. The current site URL is inserted into the text field.
  5. To allow the site, click Allow. Then, click CloseSave Changes.
  6. Close the Settings page. Any changes you've made will automatically be saved.

You are asked to download the add-on, rather than installing it

Some websites serve add-ons as downloads, requiring you to download the file to your computer, before installing it.

  • These files are usually .xpi or .jar files.
  1. Download the file to your local computer.
  2. Click the menu button Fx89menuButton, click Add-ons and themesExtensions and themes and select Extensions.
  3. To add the downloaded add-on to the list of available add-ons, drag and drop the file into the Add-ons window. The add-on is added to the list.
  4. The installation process should begin.

Download Error -228

Disabled cache

This error message may be caused when the Thunderbird cache is disabled. To re-enable the Thunderbird cache:

  1. In the Menu bar at the top of the screen, click Firefox and select Settings (select Preferences on older macOS versions).Click the menu button Fx89menuButton and select Settings.
  2. Select the Advanced icon.
  3. Click on the Network tab.
  4. In the Cached Web Content section, the size of the cache must be at least 1 MB.
  5. Close the Settings page. Any changes you've made will automatically be saved.
  1. In the Menu bar at the top of the screen, click Firefox and select Settings (select Preferences on older macOS versions).Click the menu button Fx89menuButton and select Settings.
  2. Select the Privacy & Security panel.
  3. In the Cached Web Content section, the size of the cache must be at least 1 MB.
  4. Close the Settings page. Any changes you've made will automatically be saved.

Other software

Sometimes, firewalls or spyware screeners may cause the Download Error -228 message. If you use the CA firewall, Windows Defender, or the resident option of Spybot S&D ("Teatimer" feature), you should temporarily disable the software to see if your system permits the add-on to install. For more information, see the documentation that came with your software.

Nothing happens when you click the install link

If you click an install link and nothing happens, you should verify that JavaScript is enabled. For more information, see JavaScript settings and preferences for interactive web pages.

Corrupt extension files

Corrupt extension configuration files can cause problems when trying to install an extension. Deleting these files will make Thunderbird recreate them (removing any corruption) the next time it starts. You won't lose your extensions or their settings by deleting these files.

  1. Open your profile folder:

    • Click the menu button Fx89menuButton, click Help and select More Troubleshooting Information.From the Help menu, select More Troubleshooting Information. The Troubleshooting Information tab will open.
    • Under the Application Basics section next to Profile FolderProfile Directory, click Open FolderShow in FinderOpen Directory. A window will open that contains your profile folder.Your profile folder will open.Your profile directory will open.
    Note: If Firefox displays an error after clicking Open Folder or if you are unable to open or use Firefox, follow the instructions in Finding your profile without opening Firefox.

  2. Click the Firefox menu Fx89menuButton and select Exit.Click the Firefox menu at the top of the screen and select Quit Firefox.Click the Firefox menu Fx89menuButton and select Quit.
  3. Delete the following files:
    • extensions.sqlite
    • extensions.sqlite-journal (if found)
    • extensions.ini
    • extensions.json
  4. Restart Firefox.

Conflict with other extensions

Another installed extension may interfere with the installation of new extensions. Install the new extension in Thunderbird Safe Mode as a workaround.



Based on information from