One known reason for that is if Firefox is set to automatic private browsing. In that mode, your open tabs and windows are not written to disk and cannot be restored. To check on that: Settings page > Privacy & Security panel Scroll down to the History section and check out the selector there: * '''Firefox will: Never remember history''' => this is not compatible with restoring your session (change to another setting) * '''Firefox will: Use custom settings for history''' => if there is a check in the box for "Always use private browsing mode", this is not compatible with restoring your session (uncheck the box) * '''Firefox will: Remember history''' => This setting works, so there must be some other reason Was that the problem?