hello aidansouth, you can try the following: go to the firefox menu ≡ > help ? > troubleshooting information, click on profile folder/show folder. a windows explorer window should open up with your profile folder in place - in there navigate to the sessionstore-backups folder and create a backup of the file named previous.js in a save location. then close firefox and copy the previous.js directly into the root profile folder renaming it to sessionstore,js (& replacing the file with the same name that is already there). afterwards launch the browser again...

for more information about this process you also might want to read https://dutherenverseauborddelatable.wordpress.com/2014/06/26/firefox-the-browser-that-has-your-backup/ beforehand.

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).

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