Since you are posting in the "Firefox for iOS" section I take it you were trying to install an extension onto Firefox running on a iPhone or iPad. Due to Apple's restrictions on what browser and JavaScript engines can be used the Firefox is basically a shell and not using the same codebase and Gecko like desktop Firefox and Firefox for Android does. Extensions for desktop Firefox (for Windows, Mac OS X, Linux etc) https://addons.mozilla.org/firefox/ Extensions for Android Firefox https://addons.mozilla.org/android/ There may be possibility of Extensions support for Firefox for iOS in future https://support.mozilla.org/en-US/questions/1353304#answer-1449625