I've already mailed this to the developer of Googlebar Lite, but since I'm not sure what the cause is, I wanted to ask here too.
It seems to happen when I have Googlebar Lite enabled and then visit a Wordpress site using the Atahualpa theme. If I then resize the browser, Firefox (3.6.13 but happened in earlier versions too) freezes, using 100% CPU, and sometimes won't quit. Sometimes it does recover and I'm able to increase the size of the window again. The more tabs / memory Firefox is using, the less likely it is to recover. It seems to start happening when the Googlebar Lite search box causes the address bar to reach its minimum size (hope that makes sense). The page rendering is often slightly corrupt, with elements 'jumping' left and right as if they're constantly trying to resize or re-align.
It can be cured by either (a) disabling Googlebar Lite, or (b) closing the Atahualpa themed tab. Unfortunately, I'd really like to use both!