I expect that you can find that plug-in in either /Library/Internet Plug-Ins or in ~/Library/Internet Plug-Ins and delete it from there.
Nope. In fact there are no plug-ins under any Library entry. So I presume that they're somehow embedded in Firefox without access by any apparent means and hidden from the Mac.EDIT:
I diddled around some more and found it in my Firefox folder, removed it, and that solved the problem.
So, many thanks.