I just upgraded my Kubuntu 8.0.4 desktop to the latest 8.10 Intrepid release. FireFox 3.0.5 and all other GTK based apps are having serious rendering issues and leaving artifacts.
After alot of searching, I've found a bug report for it:https://bugs.launchpad.net/ubuntu/+source/gtk-qt-engine/+bug/223274
A forum thread on the subject:http://forum.kde.org/think-importance-gtk-qt-engine-greatly-underestimated-t-8227-2.html
As a workaround in Kubuntu go to->
K-Start -> System -> System Settings -> Appearance -> GTK Styles and Fonts
Select "Use another style" and set it to "Raleigh".
Seems to cause FireFox to behave better.