Plugins don't get removed when uninstalling Firefox. Plugins remain in the location where they are installed by the program that installed the plugin, and Firefox locates those plugins thru registry settings and uses the plugins from that location outside of Firefox.

To actually remove a plugin you would have to remove the program that installed a particular program, or remove the actual files that Firefox uses or delete the registry lines for that plugin that Firefox uses to see that plugin.

See:

• http://kb.mozillazine.org/Issues_related_to_plugins#Identifying_installed_plugins
• https://support.mozilla.com/kb/Troubleshooting+plugins