Mozilla Default Plug-in which will be installed along with Firefox, you will find about this plug-in by typing about:plugins in address bar in Firefox.Wondering what this plug-in actually do, should this plug-in be enabled or disabled in Firefox, here is the explanation.
Mozilla Default Plug-in is actually plug-in Finder which will be activated and shows yellow information bar on page that requires to load missing plug-ins and asks users to install them.
For example if you don’t have Flash player plug-in installed for Firefox and when you visit YouTube or other flash content pages Mozilla Default Plug-in triggers and show information bar about “Additional plugins are required to display all the media on this page “.
You can enable or disable Mozilla Default Plug-in from Tools>Add-ons>Plug-ins and by selecting the plug-in.
By default Mozilla Default Plug-in comes enabled in Firefox so you should leave it as it is.I also have not found any significant importance to this plug-in despite this been disabled Firefox still shows information bar to install missing plug-ins.
Update : Mozilla default Plug-in removed in Firefox 4 or does not exists by default.