Unable to install add-ons or extensions

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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 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 version.
  • 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.

Software installation disabled

It is possible to disable all installation of software, including add-ons, through Firefox. To check to see if software installation has been disabled:

  1. In the Location bar, type about:config and press EnterReturn.

    • The about:config "This might void your warranty!" warning page may appear. Click I'll be careful, I promise! to continue to the about:config page.
  2. In the FilterSearch textbox, enter xpinstall.enabled.
  3. The value of xpinstall.enabled should be set to true. If this value is currently set to false, then software installation is disabled.
  4. To set the value to true, right-clickhold down the Ctrl key while you click the property name, and select Reset.
  5. Try to re-install the extension.

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. At the top of the Firefox window, click on the Firefox button and then select OptionsAt the top of the Firefox window, click on the Tools menu and then select OptionsOn the menu bar, click on the Firefox menu and select Preferences...At the top of the Firefox window, click on the Edit menu and select Preferences

    Click the menu button New Fx Menu and choose OptionsPreferences

  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. Click OK to close the Options windowClick Close to close the Preferences windowClose the Preferences window .

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. At the top of the Firefox window, click on the Firefox buttonOn the menu bar, click on the Tools menuAt the top of the Firefox window, click on the Tools menu, and then click Add-ons. The Add-ons Manager tab will open. Click the menu button New Fx Menu and choose Add-ons to open the Add-ons Manager tab.

  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. At the top of the Firefox window, click on the Firefox button and then select OptionsAt the top of the Firefox window, click on the Tools menu and then select OptionsOn the menu bar, click on the Firefox menu and select Preferences...At the top of the Firefox window, click on the Edit menu and select Preferences

    Click the menu button New Fx Menu and choose OptionsPreferences

    .
  2. Select the Privacy panel.

  3. Set Firefox will: to Use custom settings for history. Custom History Fx 5 - Win Custom History - Mac Custom History - Linux Custom History Fx21 WinXP Custom History Fx21 Win7 Custom History Fx21 Mac Custom History Fx21 Linux
  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.
    • If it is listed, click on its entry, then click Remove Site.
  8. Click OK to close the Options windowClick Close to close the Preferences windowClose the Preferences window .
  1. At the top of the Firefox window, click on the Firefox button and then select OptionsAt the top of the Firefox window, click on the Tools menu and then select OptionsOn the menu bar, click on the Firefox menu and select Preferences...At the top of the Firefox window, click on the Edit menu and select Preferences

    Click the menu button New Fx Menu and choose OptionsPreferences

    .
  2. Select the Privacy panel.

  3. Set Firefox will: to Use custom settings for history. Custom History Fx 5 - Win Custom History - Mac Custom History - Linux Custom History Fx21 WinXP Custom History Fx21 Win7 Custom History Fx21 Mac Custom History Fx21 Linux
  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.
    • If it is listed, click on its entry, then click Remove Site.
  8. Click OK to close the Options windowClick Close to close the Preferences windowClose the Preferences window .

Download Error -228

Disabled cache

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

  1. At the top of the Firefox window, click on the Firefox button and then select OptionsAt the top of the Firefox window, click on the Tools menu and then select OptionsOn the menu bar, click on the Firefox menu and select Preferences...At the top of the Firefox window, click on the Edit menu and select Preferences

    Click the menu button New Fx Menu and choose OptionsPreferences

    .
  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. Click OK to close the Options windowClick Close to close the Preferences windowClose the Preferences window .

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:

    At the top of the Firefox window, click on the Firefox button, go over to the Help menuOn the menu bar, click on the Help menuAt the top of the Firefox window, click on the Help menu and select Troubleshooting Information. The Troubleshooting Information tab will open.Click the menu button New Fx Menu , click help Help-29 and select Troubleshooting Information. The Troubleshooting Information tab will open.

  2. Under the Application Basics section, click on Show FolderShow in FinderOpen Directory. A window with your profile filesfolder will open.
  3. Note: If you are unable to open or use Firefox, follow the instructions in Finding your profile without opening Firefox.

  4. At the top of the Firefox window, click on the Firefox button and then select ExitAt the top of the Firefox window, click on the File menu and then select ExitOn the menu bar, click on the Firefox menu and select Quit FirefoxAt the top of the Firefox window, click on the File menu and select Quit

    Click the menu button New Fx Menu and then click ExitQuit Close 29

  5. Delete the following files:
    • extensions.sqlite
    • extensions.sqlite-journal (if found)
    • extensions.ini
  6. 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)



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