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This article describes various reasons that would cause an add-on not to be able to be installed and what you can do to fix the issue.

{warning}Starting with Thunderbird version 63, many changes with how add-ons work in Thunderbird may result in add-ons that are not updated by their authors to stop working.{/warning} 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. __TOC__ = Add-on is not compatible = An internal file in each add-on indicates the range of Thunderbird versions with which the extension is compatible. In the [https://addons.thunderbird.net/ 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 Thunderbird is incompatible with the add-on, you can: * Update to the latest version of Thunderbird. For more information, see [[Updating Thunderbird]]. * Find a version of the add-on that is compatible with your version of Thunderbird. The [https://addons.thunderbird.net/ 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 [https://addons.thunderbird.net/ 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 [[Add-ons that cause stability or security issues are put on a blocklist]]. = 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 {filepath [http://en.wikipedia.org/wiki/XPInstall .xpi]} or {filepath [http://en.wikipedia.org/wiki/JAR_%28file_format%29 .jar]} files. # Download the file to your local computer. # [[T:Open Add-ons|type=Extensions]] # 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. # The installation process should begin. = 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. # [[T:profileFolder]] # Delete the following files: #* {filepath extensions.sqlite} #* {filepath extensions.sqlite-journal} (if found) #* {filepath extensions.ini} #* {filepath extensions.json} # Restart Thunderbird. = Conflict with other extensions = Another installed extension may interfere with the installation of new extensions. Install the new extension in Thunderbird [[Safe Mode Thunderbird|Safe Mode]] as a workaround. <!-- MZ credit --> <br><br> '''''Based on information from * [[unable-install-add-ons-extensions-or-themes]] * [http://kb.mozillazine.org/Unable_to_install_themes_or_extensions_-_Firefox Unable to install themes or extensions - Firefox (mozillaZine KB)]'''''