I see a lot of these front pages with "major bummer, this video can't be played with your setup". Feels like the era of Flash all over again.
That should give your Gstreamer support for all formats libav supports.
I remember being surprised that sites like dailymotion and vimeo still wouldn't work since I expected firefox to come with h264 support a while back (the whole cisco thing if I remember correctly). Always just attributed it to not having flash installed or whatever and shrugged my shoulders.
Anyways they work now, and at http://www.youtube.com/html5 it showed no h264 support before, but now it does. So thanks for the comment, without which I would have still been living in the dark. :)