I've noticed this happening for quite a while now, it doesn't happen all the time, usually when I've had a page open for quite a long time, especially when I'm writing a long post on this forum!

Extremely frustrating because I lose pages and pages of stuff that I've written.
Version - Mozilla/5.0 (X11; U; Linux x86_64; en-GB; rv: Gecko/2009032600 SUSE/3.0.8-1.1.1 Firefox/3.0.8. It happens usually when I right click in order to copy text, or use the save as dialogue.

I can't switch between apps, or desktops, get the alt-f2 box to appear, or even the menu bar. If I kill the firefox process, control of KDE4 comes back.