I have an install of Debian unstable that recently had a pile of upgrades, and afterward had extremely slow scrolling in Iceweasel/Firefox/Epiphany and even Opera. I have the fglrx drivers installed. I tried safe mode for Iceweasel, but no improvement. I finally resorted to "vesa" drivers in xorg, which seems to have fixed the problem. More if I discover the cause.
Perhaps I'll try an aptitude reinstall fglrx to see what happens.
In the mean time I have installed...