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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.

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. __TOC__ = 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 [https://addons.mozilla.org/ 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 [https://addons.mozilla.org/ 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 [https://addons.mozilla.org/ 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: # [[T:aboutconfig]] # In the {for not fx11}Filter{/for}{for fx11}Search{/for} textbox, enter '''xpinstall.enabled'''. # The value of {pref xpinstall.enabled} should be set to {pref true}. If this value is currently set to {pref false}, then software installation is disabled. # To set the value to {pref true}, {for win,linux}right-click{/for}{for mac}hold down the {key Ctrl} key while you click{/for} the property name, and select {menu Reset}. # 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: {note}Firefox prevented this site (www.example<!-- -->.com) from asking you to install software on your computer.{/note} To allow the site, click {button Allow} in the yellow bar. To add the site to the trusted sites list: # [[T:optionspreferences]]. # Select the {menu Security} panel. # At the right of ''Warn me when sites try to install add-ons'', click {button Exceptions…}. # 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. # To allow the site, click {button Allow}. Then, click {button Close}. #[[T:closeOptionsPreferences]]. = 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. = 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. {for not fx22} # [[T:optionspreferences]]. #[[T:customhistory]] # Make sure '''Accept cookies from sites''' is check marked. # Make sure '''Accept third party cookies''' is check marked. #* [[Disable third-party cookies in Firefox to stop some types of tracking by advertisers|Disabling third party cookies]] can cause problems with some websites. Enable this setting to rule it out as a cause for your problem. # Click {button Exceptions…}. # Make sure the site you're trying to access isn't listed. #* If it is listed, click on its entry, then click {button Remove Site}. #[[T:closeOptionsPreferences]]. {/for} {for fx22} # [[T:optionspreferences]]. #[[T:customhistory]] # Make sure '''Accept cookies from sites''' is check marked. # Make sure '''Accept third party cookies''' is set to '''Always'''. #* [[Disable third-party cookies in Firefox to stop some types of tracking by advertisers|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. # Click {button Exceptions…}. # Make sure the site you're trying to access isn't listed. #* If it is listed, click on its entry, then click {button Remove Site}. #[[T:closeOptionsPreferences]]. {/for} = Download Error -228 = == Disabled cache == This error message may be caused when the Firefox cache is disabled. To re-enable the Firefox cache: # [[T:optionspreferences]]. # Select the {menu Advanced} icon. # Click on the {menu Network} tab. # In the Cache section, the size of the cache must be at least '''1 MB'''. #[[T:closeOptionsPreferences]]. {for win} == 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. {/for} = 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. # [[T:profileFolder]] # [[T:closeFirefox]] # Delete the following files: #* {filepath extensions.sqlite} #* {filepath extensions.sqlite-journal} (if found) #* {filepath extensions.ini} # Restart Firefox. = Conflict with other extensions = Another installed extension may interfere with the installation of new extensions. Install the new extension in Firefox [[Troubleshoot Firefox issues using Safe Mode|Safe Mode]] as a workaround. <!-- MZ credit --> <br/> <br/> '''''Based on information from [http://kb.mozillazine.org/Unable_to_install_themes_or_extensions_-_Firefox Unable to install themes or extensions - Firefox (mozillaZine KB)]''''' [[Template:ShareArticle|link=http://mzl.la/N7XgX6]]