As per my comment on the question, mozilla provide some instructions for manually uninstalling.

Manually uninstalling a plugin

If you can't use an uninstaller program to remove a plugin, you can
remove it manually:

1. In the Location bar, type about:config and press Enter.

   
   
   
   
   
   • The about:config "This might void your warranty!" warning page may appear. Click I'll be careful, I promise!, to continue
     to the about:config page.
   
   



2. Search for the preference: plugin.expose_full_path.




3. Double-click on the plugin.expose_full_path preference in the list to change the value to true.




4. Enter about:plugins into the Location bar to display the About Plugins page.




5. Each entry in the About Plugins page will have "File name:" followed by a path. Use Windows Explorer to navigateNavigate to the
   folder shown for the plugin you want to remove.




6. Rename the file to something other than its normal name - e.g. npswf32 becomes Xnpswf32




7. Double-click on the plugin.expose_full_path preference in the list on the about:config page to change the value back to false to
   avoid exposing the plugin path to websites.

The plugin will be removed.

_{The above Mozilla.org community-contributed content is made available under CC-BY-SA 3.0, which permits properly-attributed derivative works distributed under the same license. CC-BY-SA is also the license that covers content here on Ask Ubuntu. Its formatting is changed very slightly to accommodate the StackExchange format. The page from which the above block-quote was taken lists Bo102010, cor-el, Verdi, and scoobidiver as contributing authors.}