Solved the issue.
Inspired by comments from https://addons.mozilla.org/de/firefox/r ... display/39
I visited http://www.mydigitallife.info/2008/06/0 ... -disabled/
and followed workaround 3. In my situation,
grep em:name */install.rdf
revealed that some folders did not contain an install.rdf at all and thus seemed not to belong to any extension. I simply deleted those folders. Restarted firefox, installed mouse gesture redox, restarted when prompted to - voila.
Workaround 3: Uninstall manually user-specific extensions, toolbars, themes and add-ons
If all methods fail to remove or uninstall an extensions, user can manually uninstall an add-on using steps below, which work for user-centric add-ons:
1. Open the Firefox profiles folder, which normally located at %AppData%\Mozilla\Firefox\Profiles\xxxxxxxx.default\, where xxxxxx is a random string. User should pick the profile folder (if inside Profiles folder there are multiple sub-folders) that are active, i.e. profile for user account to uninstall the extension.
2. Open the extensions folder inside the profile folder.
3. Each folder here inside the “extensions” folder corresponds to an extension. Some folders may have readable name that resembles the plugin, extension or toolbar, and can be easily recognized. Often the names of the folders are obscured. To identify which extension or plug-in the folder is representing, go to each folder and perform the following steps:
1. Use a text editor (e.g. Notepad or alternative text editor) to open the install.rdf file inside it.
2. Search for the following text:
3. Locate the folder with install.rdf file with the name after the “em:name” corresponding to the extension intends to remove or delete.
4. Delete the whole folder for the extension.