Wait... that's a FireFox extension, not a plugin.

Uninstall the extension and restart FireFox. See if it still gives you the message.

1) If you don't see that message again, it was that extension giving you problems.

2) If you uninstall that extension and still see that message, you screwed something up in FireFox. I would back up your bookmarks and uninstall FireFox and re-install a fresh copy.

Try step 1 first.