After installing the latest K/Ubuntu the integration with the KDE 4 is far far better and I started to use it in my daily work.
But I soon noticed 2 problems with Firefox in KDE 4.
When selecting a large chunk of text and then try to move it around, Firefox and the whole system freezes. If you have selected a little text, then it will return to normal after a while. If a large chunk then you may not have the nerves to wait for it to return to normal.
I filed this here, because I'm not sure where to go - KDE or firefox...
The Mouse Gestures Redox addon in Firefox was trashing the browser when you make a gesture. Luckily the new nightly build fixed this issue.
This is for now... and again KDE 4 ROCKS with the improved integration (maybe thanks to Ubuntu) :)