Re: Firefox is asking to update a plugin, but can't find it. As far as I know there isn't any. Why is it asking in the first place?
while clicking on 'install plug-ins', a dialog box of Plug-in Finder Service opens in an another window. Now at the bottom of that dialog box you will find a noting i.e.: 'Find out more about plugins or manually find missing plug-ins' click on that and you will find your Plug-in Details, Status & Action check that whole thing and if it shows that you have to update it then perform the needed action.
Also will like to say that you should use the latest version of "Mozilla Firefox"