After following this excellent support thread http://ubuntuforums.org/showthread.php?t=1980663 I got my ATi card working ok, but now I'm facing great performance issues in Firefox (12.0) in plain html pages (like this one), nothing about flash or animations or videos.

When scrolling the page up and down the scrolling gets pretty slow! Also this issue appears when browsing HTML5 pages like games, it get's really slow!

Thanks in advise!