How to restore Firefox sessions if Session Restore is not working correctly

"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