I thought it was sessionstore.bak that would have the previous session.

What is it you are trying to restore?

My session which I lost after many crashes

If you use the Time Machine then try to restore a sessionstore.js file from that backup.

• http://www.apple.com/support/timemachine/

You can use this button to go to the currently used Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)

• https://support.mozilla.org/kb/Recovering+important+data+from+an+old+profile