I've noticed this and done some experiment... I cannot tell you exactly why but I think this has to do with the way browsers detect which pages should be hardware-accelerated and somehow interpolate between dom element movement.

One way I found to generate the same effect for firefox (and probably safari though I didn't test it) is to double the refresh rate. If you set 15 ms instead of 30 you'll see that everything get much smoother. You'll have to change some value so that the resulting game doesn't get speeded-up.