I don't understand, It seems HTML5 video to me. What kind of setup do you have?
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. :)