Check about:sessionsrestore

If a session is lost, hopefully the restore pages open automatically, but if not go to about:sessionrestore in the location bar. If this page is empty, continue reading.

Your page should look like this :

Restoring from a backup file

1. Go to about:support in your location bar and click on the Open FolderShow in Finder button in the Profile Folder line. A filesystem view of your profile folder will show up.

   

2. In the nhtg6jgg.default-release folder, if you scroll down a bit, you should see the sessionstore-backups folder.
3. Make a copy of the sessionstore-backups folder.
4. Shut down Firefox. Generally you want to avoid filesystem operations (move, copy, delete) in this folder while Firefox is running. The copy of the unaccessed backups folder was necessary and should be safe.
5. Move the file called sessionstore-backups.jsonlz4 to another location (like your deskptop). Moving rather than deleting is a precautionary step.

   Note: This file does not exist while Firefox is running.

6. Navigate to the sessionstore-backups backup folder you made.
7. Review the files in this folder by date modified (you can ignore their names, any file in this folder should be restorable). If the most recently modified file is particulary small in size compared to others, it may be mostly blank and you'll want to use the bigger one.
8. Copy your chosen file into the main profile folder.
9. Rename the file to sessionstore.jsonlz4.
10. Start up Firefox.

Conclusion

If you feel that you are more prone to losing your session - sessions with lots of tabs may fall into this category - or would just like insurance, the Tabs Session Manager helps keep sessions safe.