If neither your old session nor the Restore Session error page appears after a crash, or you accidentally click on “Start New Session”, you can enter about:sessionrestore in the Location Bar (address bar) to force the Restore Session page to appear, after first restoring a backup file to restore the old session data. The session data is stored in sessionstore.js; this file can be restored (while Firefox is not running) from backups, including "sessionstore.bak". The file "sessionstore.bak" is created if you launch Firefox after a crash has occurred. It is a copy of the current sessionstore.js that gets lost otherwise, if you restart Firefox.