Restoring session from sessionstore.bak file

Firefox crashed and this time it lost all my tabs (about 50 of them). This isn't the first time and so immediately I copied the .bak file and then hit system restore and unfortunately found only one restore point. It said the system restore was successful, but when I opened firefox again, nothing's there! I copied the .bak file (4 mb) to the profile folder, deleted the session.js (1 kb) and then renaming the .bak file to session.js and nothing again! What do I do!