Awesome! I didn't know FF supported it yet. Are you sure that FF isn't launching a 3rd party app, like Luke suggested? If it truly is being rendered by the browser and not a proprietary plugin (even if it IS MPlayer :-P) then that's awesome.
As a side note - fired up the link in IE7 - it plays but it loads in a java player. Not sure if it's native to the browser or if they've actually written a player. Either way, cool.