Possibly related?: scroll down, then up. Before it reaches the top position, it locks up Firefox Aurora (current nightly build). Possibly a bug in Firefox, but those are relatively rare, even in the nightlies.

edit: specifically, here: https://www.dropbox.com/s/abbuk5ktjpmos7f/Screenshot%202013-... and you have to be scrolling in individual steps (since scrolling has inertia, can push you >1 step).