Aye, but on Mac, Firefox doesn't fully hardware accelerate the draw operations.
Did find this quote from Opera though which probably explains their stance: http://my.opera.com/...-acceleration-2
Ahh, that explains things a bit better. They're writing their own 2D drawing API like Direct2D, but one that uses OpenGL underneath. I know Mozilla have similar plans, but that's a long way off (although some of their current drawing work is a precursor)
It'll be interesting to see what Opera can achieve, performance and capability wise (e.g. Direct2D is missing a few things you need for 100% GPU accelerated <canvas>, IE9 simply ignored those bits while Mozilla implemented them via OpenGL/Direct3D blending)