I've had similar experiences with graphical corruption in X, and Firefox was almost always involved. Same radeon driver as you. And it seems to be distro-agnostic: I've seen it with Arch, Gentoo and Mint. All I know is that it's a recent thing: if I run an older version of my Gentoo build, it doesn't happen. But otherwise I see the same results.

I have no idea what to point to, because it's really weird. Scrambles /everything/ that goes through video. (And until now, I figured it was a quirk of my bizarre hardware. Guess not.)