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 Updating Firefox.
• 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.

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 Filter 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:

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 (Tools menu in Windows XP) and then click 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 PreferencesAt the top of the Firefox window, click on the Tools menu and select Options...On 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...
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.

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. In the menu at the top of the Firefox window, click the Tools menu, and select Add-ons. The Add-ons window is displayed.
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 (Tools menu in Windows XP) and then click 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 PreferencesAt the top of the Firefox window, click on the Tools menu and select Options...On 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...
2. Select the Privacy panel.
3. Check mark Accept third-party cookies.

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 (Tools menu in Windows XP) and then click 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 PreferencesAt the top of the Firefox window, click on the Tools menu and select Options...On 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...
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.

Unexpected installation error -203

See Unexpected installation error -203 when installing add-ons.

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.

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:

   1. At the top of the Firefox windowOn the menu bar, click on the Help menuAt the top of the Firefox window, click on the Firefox button, go over to the Help menu (on Windows XP, click on the Help menu)On 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.
   2. Under the Application Basics section, click on Open Containing FolderShow in Finder. A window with your profile filesfolder will open.
      Note: If you are unable to open or use Fire​fox, follow the instructions in Finding your profile without opening Firefox.

2. At the top of the Firefox window, click on the Firefox button (File menu in Windows XP) and then click 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 QuitAt the top of the Firefox window, click on the File menu and 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.
3. Delete the following files:

• • extensions.ini
  • extensions.cache
  • extensions.rdf

1. Restart Firefox.

Add-on is blocked

Certain add-ons have been blocklisted by Mozilla for being unsafe. For more information see, Add-ons Blocklist.

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