Look inside the extensions directory inside the Mozilla Firefox directory. If you installed Firefox to a custom directory on your system, you will find it there instead. On 32-bit versions of Windows, you will find it in C:\Program Files\Mozilla Firefox.
By default, Firefox is installed to C:\Program Files (x86)\Mozilla Firefox on 64-bit versions of Windows. The first place to look when an extension can’t be uninstalled from within Firefox is in Firefox’s installation directory. In some cases, an unscrupulous extension may not add any entry to Programs and Features at all, attempting to hide itself and prevent you from removing it from your system.
In the example above, the extension can only be removed by uninstalling the complete Logitech SetPoint software package. Just perform a search for the name of the extension and uninstall it as if it were any other program.
If the globally installed extension is a well-behaved piece of software, you’ll probably be able to uninstall the extension from the Programs and Features window in the Windows Control Panel.