I also have Firefox opening popups on reload sometimes (both on Fedora Core 5 and Core 6) and crashig when I close them. Sometimes it is an unwanted popup, somtimes just the reloaded windows got opened in a deadly popup.
It is frequent in average once a day (I have many reloads, for example always a tab on CNN, frequently reloading).
I use and I have always used on both machines proprietary nvidia drivers.
Enzo Marinari