intermitent shutdown apparently due to firefox

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http://askubuntu.com – It is happening with 12.04 (just upgraded my system): when firefox is open the system may shutdown spontaneously. All jobs stop working, the opening screen appears asking the password as if the machine was just rebooted. After re-opening and restarting firefox the message says Firefox is having problems to recover tabs possibly due to one of the open webpages. Any clue ? (HowTos)