O'Callahan, one of Firefox' lead developers, is very clear that he isn't "here to bash IE9." However, he is clearly upset about the way Microsoft markets IE9 (that may be somewhat reminiscent to 1997, since Netscape wasn't quite happy about Microsoft and its IE4 marketing back then either.
According to O'Callahan, Microsoft's "message that IE9 is the apex of what a browser can do with the GPU is nonsense." He explains that Microsoft's claims of full web platform hardware acceleration are ridiculous, especially since there are examples in which Firefox trumps IE's hardware acceleration engine and IE9 lacks key web platform features. In O'Callahan's words: "Microsoft's disdainful statement that '[other browsers are] choosing to accelerate only pieces of the Web platform' is hypocritical, because that's exactly what they do themselves."
As browser user, you are left standing in the rain as we know all too well that browser benchmarks and results can be shifted in any direction you want: Simply use the benchmarks that take you to your goal. Websites such as html5test.com and whencaniuse.com indicate that Mozilla and Google have, in fact, the edge in HTML5 support, but there is now way that we can tell which is the browser with the faster hardware acceleration engine. Mozilla shows off one example in which Firefox 4 RC1 is about ten times faster than IE9 RC, but IE9 RC crushes Firefox in Microsoft's own acceleration tests. It's up to you whom you believe at this point as there is no standardized and generally accepted hardware acceleration test.
We tend to think that IE9 is a big step forward and Firefox is a giant leap for Mozilla. Until we have a standardized test, we are happy with that.