I found a solution that other people might find helpful: Short version: Do a disk check to fix the error - requires a reboot for the disk to be fixed if the profile is on your windows partition. Long Version: It was a disk corruption that for some odd reason was continuously replacing the sessionstore file with a corrupt version timed and dated at a time when firefox had crashed. The fix was to go to 'my computer' -> Right click the drive with the profile on -> tools tab -> [check now] make sure [automatically fix file system and errors] is checked Click [start] Click [force dismount] Click [schedule a check] *Reboot computer* Let the disk check fix the error. Fixed (if not then close firefox, delete sessionstore.js in the profile folder and restart firefox).