You can disable plug-ins from Tools > Add-ons > Plugins or open a new tab and type about:plugins. The plugin list will load with file name and its corresponding path. Use Windows Explorer or any other file manager and browse to the location and rename plugins you want removed. Just add X in front of the filename.
"C:\Program Files (x86)\Google\Update\18.104.22.168\npGoogleUpdate3.dll" plugin becomes XnpGoogleUpdate3.dll. Next time you'll open Firefox, the plugin will be deleted.