Another thing to try is to copy your firefox profile off and start with a clean profile. Make a copy of the
directory in your /home/<user> or rename that directory to
, then restart Firefox to see if the problem persists. If the problem goes away, it is something in your old profile. Unfortunately at that point, you will have to restore your bookmarks, cookies, etc.
An alternative could be to start firefox from a terminal and use it until it crashes. Check the terminal for output to see if there is anything meaningful.