You can 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).

Thank you. I managed to do so and now I can access my old Restore Session option.

But the glitch remains; as in, the Restore Session Error window shows all my active tabs, but if I click restore, it opens a new window with all empty tabs.

If it helps clarify the issue, I get at least two Unresponsive Script errors when Firefox tries to load these tabs. One relates to the tabbrowser and the other to content. These did not appear before the blackout.

Is there any way to resolve this issue?

Edit: I opened the previous.js file I was using to restore my session in Word and all the URLs are definitely there. So I am not sure why Firefox is only opening empty tabs.