As far as I can see setting the priority in which plugins handle a piece of media when more than one plugin is capable of doing so in firefox is undocumented and there is no way to do so at about:plugins. This was rather irritating as the gxine starter plugin is nowhere nearly as good as the totem plugin, yet it handles more mime types. So it would make a great backup plugin. So how is priority determined? Simple, the most recent plugins are used in preference to older ones, again this isn’t documented.

So, in *nix all you should have to do is open up a terminal window and type:
cd /usr/lib/firefox/plugins
sudo touch -d 01/01/95 <lower priority plugin>
In my case <lower priority plugin> was gxineplugin.so.

In windows you can download eXpress TimeStamp Toucher, open it and choose an old date and browse to the lower priority plugin which should be in C:/Program Files/Mozilla Firefox/Plugins/ by default.

You can confirm that it has worked by going to about:plugins in firefox and making sure your lower priority plugin is at the furthest down the page of the plugins.

Comment and let me know if this works for you too.