I have tried Glitch on safari/mac on three computers. The first two stopped working after the Great Silence, and the other one, which I only got recently, works fine. They are all 5.0.3/10.6.5
Updating this. Yesterday glitch worked fine on Safari on my mac. Today, neither the arrow or keyboard letters would let me move. I can move only with the mouse clicking on the screen, although I can jump with the space bar. It's difficult to jump and move this way (just the mouse clicking on the screen). Working fine on FF.
I've also had intermittent problems using Safari, similar to the sort of thing Tibbi describes. Doesn't seem to have any rhyme or reason to it. So far, I've always been able to use Firefox when Safari gets, uh, glitchy... so it's not a problem with the game or my connection.