“What many Firefox users do not know is that Firefox saves two files with Session Restore information to the user profile:
- sessionstore.js
contains information about the current session. If you monitor the file
you will notice that its size increases and decreases during a browsing
session.- sessionstore.bak which contains session information from the previous session.
Source: Ghacks Technology Newsletter
Details 
How to restore Firefox sessions if Session Restore is not working correctly
