You are on a Mac, so if you use the Time machine then you could try to restore an older version of the sessionstore.js file in the Firefox profile folder.

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

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

You can look at the Session Manager extension to manage multiple sessions for future prevention.

• https://addons.mozilla.org/firefox/addon/session-manager/

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.

You can backup the session files in the sessionstore-backups folder in the Firefox Profile Folder to make sure not to lose possible important session data.

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).

Sadly, I hadn't used a TimeMachine backup in over a month. I suppose I am doomed now to look through 6 months of history. Ugh.

Hi nd-anchoress, I don't know whether you followed the rest of cor-el's post after the first paragraph. Did you look inside the sessionstore-backups folder to see what files are in there, possibly with a recent date? Usually people who have a lot of tabs open tend to have had them open for quite a while so a file a few days or weeks older might still be better than nothing.