Make sure the plugin is recognized by Firefox. Type about
lugins in the address bar and see if a line: mplayerplug-in is now gecko-mediaplayer 0.9.8 is there and you've got all the supported video types listed below.
If it's there then you should be good to go.
Open your ~/.mplayer/config and put this in it:
And finally run gnome-mplayer open the prefs panel and tick every but the last option on the "Plugin" page.