Firefox will always use the sessionstore.js file when present to restore the previous session.

Does the sessionstore.js file get updated?

Did you check "History > Recently Closed Tabs/Windows" ?

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

• Help > Troubleshooting Information > Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Firefox uses the sessionstore.js file in the profile folder to store session data, but this file is only present when Firefox is closed. The sessionstore.js file is created from recovery.js when you close Firefox and is removed and copied to previous.js when you start Firefox to make is possible to restore the session at any time.

Backup the session files in the sessionstore-backups folder in the Firefox Profile Folder to make sure not to lose possible important session data. Do NOT close Firefox when Firefox is already running.

You will normally find these files in the sessionstore-backups folder:

previous.js (cleanBackup: copy of sessionstore.js from previous session that was loaded successfully)
recovery.js (latest version of the sessionstore written during runtime)
recovery.bak (previous version of the sessionstore written during runtime)
upgrade.js-<build_id> (backup created during an upgrade of Firefox)

You can copy a file from the sessionstore-backups folder to the main profile and rename the file to sessionstore.js to replace the current file (make a backup copy of the current sessionstore.js).

See also:

• http://www.ghacks.net/2014/06/27/mozilla-launches-improved-session-restore-firefox-33/

Well, it works now.

I even removed the now working sessionstore.js to reload the previous.js and restore.js and those both work just fine now.

And looking though the recycle bin, I can also guarantee that there were not typos in naming the files.

I have no idea what could have been causing this problem.

The same issue has recurred and did not magically solve itself like before.

Here is a better solution using the Session Manager addon.

To modify an existing sessionstore file into a Session Manager one you need to do the following:

1. Put the sessionstore file into the folder pointed to by: Session Manager/Open Session Folder 2. Use a text editor to copy the first four lines of an existing .session file into your sessionstore file 2. Replace file extension with .session

Then it should be possible to load the session.