I can barely scroll with the animation turned off, on a 2.8GHz CPU (it only returns to normal when reloading the page with js disabled). It's times like this when I really appreciate NoScript, but the pledge link only appears with js enabled. The fancy background could be hurting you in the pocket.
Anyway, I've made it so that the bitmaps are hidden when the animation is disabled, since they are the most likely candidate for making scrolling slow.
That works, clicking "animation" restores Firefox back to normal. What's weird though, is that the usual slow script warning doesn't work here. That shouldn't happen and might be a regression. And no to both, although it's probably slower than yours. Even with 170x140 and the main div removed (responsive design view), it won't animate.
The slow script warning, as far as I understand it, only fires when the script runs uninterrupted for a certain time. This was a case of animation frames taking up more time than their framerate, and thus firing end-to-end, killing the responsiveness of the browser itself.
