Check out the following support page: https://support.mozilla.org/en-US/kb/cannot-uninstall-an-add-on#w_uninstalling-manually If that doesn't help try reading the following sections of the mozilla knowledge base on uninstalling extensions: http://kb.mozillazine.org/Uninstalling_extensions#Corrupt_extension_files http://kb.mozillazine.org/Uninstalling_extensions#Windows_Registry_extension -- I have the Avast! Online Security add-on on my Firefox (64-bit Windows 8, v 31.0) and the add-on file for that is not found in the user profile extensions folder or in the programs files extension folder. It is listed under 'Extensions' in the trouble shooting page referred to on the page linked at the top of this post as: avast! Online Security 9.0.2021.112 true wrc@avast.com I found a reference to it in the registry (using 'regedit') under HKEY_LOCAL_MACHINE\Software\Wow6432Node\Mozilla\Firefox\Extensions (Only on 64-bit versions of Windows) * as indicated by the mozilla knowledge base page 'Uninstalling_extensions' in the 'Windows_Registry_Extension' section. It looks like this: wrc@avast.com REG_SZ C:\Program Files\AVAST Software\Avast\WebRep\FF Deleting the corresponding registry entry for your extension may resolve the problem at least temporarily assume it does't try to re-register it. You may also try removing the .xpi file found at the specified (or the whole folder that is referenced in the registry as is the case in my example -- it seems to a folder which might be equivalent to an unpacked .xpi file).