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:

• Update 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.
• Contact the author of the add-on at the support site listed on its add-on page. (Note: posting a review is not effective and will not reach the author.)

Firefox has prevented this site from installing an unverified add-on

Firefox protects you against malware and browser hijackers by blocking third-party add-ons that are not digitally signed and verified by Mozilla. For more information, see Add-on signing in Firefox.

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

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:

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 and select Settings.
2. Select the Privacy & Security panel and go down to the Permissions section.
3. At the right of Warn you when sites try to install add-ons, click Exceptions….
4. In the Allowed 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 Save 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 , 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.

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 , 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 and select Exit.Click the Firefox menu at the top of the screen and select Quit Firefox.Click the Firefox menu 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 Troubleshoot Mode as a workaround.

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