Interesting. It looks like FF4 is known to support all HTML5 and IE9 has 5 unknowns. As far as I knew, IE9 was supposed to support hadrware acceleration and have plugins for video and audio. But the one thing is that FF 4 Beta 6 doesn't support mpeg 4 or at least not yet.
I think in the long run HTML5 will kill off Flash. Even though Flash will have a niche where the power can be used (such as animations like where it started anyways) most common things like video and audio will move to HTML5 since it wont require people to have a thrid partty app that could possibly crash or cause issues.