A possible cause is security software (firewall) that blocks or restricts Firefox without informing you about that, possibly after detecting changes (update) to the Firefox program. Remove all rules for Firefox from the permissions list in the firewall and let your firewall ask again for permission to get full unrestricted access to internet for Firefox. See [[Server not found]] and [[Firewalls]] and http://kb.mozillazine.org/Firewalls See also http://kb.mozillazine.org/Browser_will_not_start_up