On the current Firefox GA Release (13) the frame rate with 80 blocks often drops briefly to 20fps, which is not great. But on Firefox Nightly (16), I get very smooth frame rates, very close to 60fps! I see similar results in Chrome Dev (21) as well . So it looks like it is now possible to run games on the web with lots of 2D physics in them.
 On Opera 12 I manually enabled WebGL, but sadly the page doesn't render properly.