Tried this with my ATI Xpress 1100 and a 720p anime episode from Sayonara Zetsubou Sensei.
It didn't flicker like usual with other releases, but SMPlayer's controls were messed up, the video playback was way too slow, and X server crashed after 10 seconds
Also trying to play anything on Wine would hang up the PC completely. Not sure if it's this release only though, since I havent tried wine with fglrx for so long.
Agreed, Firefox on my nvidia card is almost unusable. I'm not sure if its hardware or a Fedora thing (I use Ubuntu and Gentoo on my other boxes), but running 3.1 nightly builds with tracemonkey enabled and "smooth scrolling" disabled made it usable, but scrolling on Twitter, etc is still painful.
I have an older nvidia card (forget the model) in my workstation, and Firefox has been alright, but not great, with binary drivers in both Ubuntu and Gentoo. Never had a problem on my main desktop with a Radeon HD 4850 (with any driver).
Edit: All of the above applies with and without compositing, except with the open source drivers on the ATI card where compositing obviously isn't an option.