For posterity, fiddling around with information from here and here, I managed to get IceWeasel to perform at 25-30 FPS by forcing Option "AccelMethod" "sna" in my xorg.conf.

Also noticed IceWeasel was a lot snappier on a fresh user account, so probably need to close a ton of the tabs I have open (doesn't affect FPS performance though once IceWeasel's all loaded).