It seems to be capped at 60FPS. But it's the simplest demo, my results for the others are nowhere near 60FPS, most aren't even usable unfortunately.
I had really bad crashes (high pitch noise, bright white flash, driver killed) on Firefox 4 and Chrome 7 when they started to use HW acceleration and problems went away after the update.
Though this was on Windows 7, no idea about OSX.
I guess that has to do with hardware acceleration. FF3.x does not have that yet, so it's falling back to software rendering SVG's, and software rendering is.. you know, awfully slow.
Try the FF4 beta if you want to see how things shape up, I think that has HW acceleration built in, though I'm not sure how far they are with WebGL.
and out of curiosity i tried a froyo powered device (droid 2.2) and got up to 26 fps.