Since I upgraded to Firefox 1.5, I've noticed that anytime I'm in a thread on the server side and I leave my browser idle for about 5 minutes, Firefox crashes. I've reproduced this on multiple machines (all Windows XP) and talked to other people that have seen the same behavior (also Firefox 1.5 on XP). I sent feedback to the Firefox team, but you might want to see if you can work around the issue on your side as Firefox may not have a timely fix.