Unable to install add-ons (extensions or themes)

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  • Revision id: 91345
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  • Creator: Lan
  • Comment: Removed outdated info
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  • Reviewed by: AliceWyman
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An inability to install an extension can be the result of problems with the extension, Firefox preferences, or the version of Firefox 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 Firefox Add-ons area of Mozilla, you can review version compatibility information on the page for each available extension.

If your installed version of Firefox 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:

  • Upgrade to the latest version of Firefox. 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 Firefox 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 Firefox Add-ons.

Add-on is blocked

Certain add-ons have been blocklisted by Mozilla for being unsafe. For more information, see Add-ons that cause stability or security issues are put on a blocklist.

Firefox prevented the site from asking you to install software

By default, Firefox 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:

Firefox 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 then select Preferences or Settings, depending on your macOS version.Click the menu button Fx89menuButton and select Settings.
  2. Select the Security panel.
  3. At the right of Warn me when sites try to install add-ons, click Exceptions….
  4. In the Allowed Sites - 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 Close.
  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 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.

Invalid file hash (possible download corruption) -261

This error message may be caused when you have Firefox set not to allow third-party cookies. To prevent this error, you need to temporarily enable third-party cookies.

  1. In the Menu bar at the top of the screen, click Firefox and then select Preferences or Settings, depending on your macOS version.Click the menu button Fx89menuButton and select Settings.
  2. Select the Privacy & Security panel and go to the History section.

  3. In the drop-down menu next to Firefox will, choose Use custom settings for history.
  4. Make sure Accept cookies from sites is check marked.
  5. Make sure Accept third party cookies is check marked.
  6. Click Exceptions….
  7. Make sure the site you're trying to access isn't listed here.
    • If it is listed, click on its entry, then click Remove Site.
  8. 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 then select Preferences or Settings, depending on your macOS version.Click the menu button Fx89menuButton and select Settings.
  2. Select the Privacy & Security panel and go to the History section.

  3. In the drop-down menu next to Firefox will, choose Use custom settings for history.
  4. Make sure Accept cookies from sites is check marked.
  5. Make sure Accept third party cookies is set to Always.
    • Disabling third party cookies can cause problems with some websites. Always accept third party cookies to rule it out as a cause for your problem.
  6. Click Exceptions….
  7. Make sure the site you're trying to access isn't listed here.
    • If it is listed, click on its entry, then click Remove Site.
  8. Close the Settings page. Any changes you've made will automatically be saved.

Download Error -228

Disabled cache

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

  1. In the Menu bar at the top of the screen, click Firefox and then select Preferences or Settings, depending on your macOS version.Click the menu button Fx89menuButton and select Settings.
  2. Select the Advanced icon.
  3. Click on the Network tab.
  4. In the Cache 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.

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 Firefox 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 FolderDirectory, click Open FolderShow in FinderOpen Directory. A window will open that contains your profile folder.Your profile folder will open.
    Note: 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
  4. Restart Firefox.
  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 FolderDirectory, click Open FolderShow in FinderOpen Directory. A window will open that contains your profile folder.Your profile folder will open.
    Note: 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 Firefox Safe Mode as a workaround.



Based on information from Unable to install themes or extensions - Firefox (mozillaZine KB)