I have three Firefox Plugins which are disabled, but no means to remove same, and they do not show up Microsoft Uninstall or Change a Program Panel.
The Plugins is a software libraries and almost related to a software installed on the operating system So it's not related to Firefox it self because the software uses a standard plugins path to Firefox plugins folder or registry and Firefox decide it'll use it or not.
You can set the plugin.expose_full_path pref to true on the about:config page to see the full path of plugins on the about:plugins page.
It is best not to leave that pref set to true as it exposes that full path to web servers via the navigator.plugins object, so reset that pref to false after you are done with the about:plugins page.
See "Manually uninstalling a plugin":
I found out about firefox and the RealPlayer plugin blocking flash drives from ejecting. I tried to uninstall the plugings on version 21 but even though I set plugin.expose_full_path to true it is not showing the path in about:plugins, only the file name. How do I get firefox to display the full path so I do not have to do searches in Windows explorer for each pluging?
The about:plugins page is broken only in Firefox 21, but works in Firefox 22 and later to show the full path of all plugins whether enabled or disabled.
You should leave the plugin.expose_full_path set to false to avoid exposing the full paths of plugins to web servers via the navigator.plugins array.
To reiterate what cor-el said, in Firefox 22 the about:plugins page does show the full path to each plugin, there is no need to change any preferences to see that path.
Locking this thread as this topic was started for Firefox 19.0, and Firefox 22.0 is the current release version.