Works fine for me on Chrome latest dev channel and Safari 5.0.2 on Mac. That said, I've had issues with Firefox 3.6.x with HTML5 in the past, ranging from outright incompatibility to extremely slow performance. Firefox 4.x supposedly fixes all those issues. YMMV.
Try Chromium. I think Firefox 3's JavaScript speed is too slow for intensive applications like this. The FF4 betas are probably much faster, but if you want to see this in a stable browser it should work fine in Chromium.
