Using the trunk firefox-cocoa build (20060531), Firefox suddenly crashed, whilst in the background. I had VoiceOver on - not sure if that was related to the crash.
I've crashed 2-3 more times today with this same stack trace. Mostly when trying to quit. Firefox hangs for a while, and then crashes.
Probably fixed with bug 339371 *** This bug has been marked as a duplicate of 339371 ***