Nah, it's the Firefox smoke and mirrors game
Firefox 4 has all the hardware acceleration features that IE9 has, except that Composition Acceleration is currently disabled by default, presumably because it's buggy as heck. This is why IE9PP3 wipes the floor with FF4b5 in both the IE and FF hardware acceleration tests in the default configuration.
I'm glad both browsers will presumably have fully working implementations by launch, it'l be good for the web overall. But to claim Firefox was fastest and first, when it clearly isn't either, is a bit of a joke.