The Mozilla Default Plug-in

By default, Mozilla Firefox comes with the file npnul32.dll in the \Plugins\ directory. If you enter about:plugins in the location bar, you’ll see a list of installed Plug-ins, with file information, and MIME-type assignments. In that list, you’ll see npnul32.dll listed as “Mozilla Default Plug-in”, assigned to the MIME-type *.
So what is the Mozilla Default Plug-in? What does it do? Why is it there? Can one disable it? How does one disable it?

The Mozilla default plug-in is the plugin finder that gets initiated, when you try to load a page that requires a plug-in, which is not found. Whether or not the default plug-in is enabled or disabled dictates how the plugin finder works.
For instance, if don’t have the Macromedia Flash plug-in, then go to their test page, this is what you’ll get with the default plug-in disabled:

This is what you’d get, if the default plug-in is enabled (click on it for full size):

To enable the Mozilla Default plugin, use about:config to change the pref plugin.default_plugin_disabled to false