Type about:support in the location bar.

Under "Application Basics", click on the Profile Directory: "Open Directory" button.

That'll open up the folder containing all browser settings, info, etc. -- the "profile directory".

Copy the entire profile directory into a new directory -- don't want to accidentally corrupt things or overwrite existing session storage data.  That also gives you a clean-ish slate to revert to.  Now close Firefox.

Copy the entire profile directory into *another* new directory.  Just to be safe.  (The profile directory when running has different data from after shutdown, because session info gets stored for backup at shutdown.)

Because session info's kind of important, there are a gazillion backups of it: "sessionstore.js", "sessionstore.bak", "sessionstore-backups/previous.js", "sessionstore-backups/recovery.js", "sessionstore-backups/recovery.bak", and a bunch of "sessionstore-backups/upgrade.js-YYYYMMDDHHMMSS" files.

The backup data you want is *probably* in "sessionstore-backups/upgrade-js-<the most recent YYYYMMDDHHMMSS that's there>".

In the profile directory, the file containing the most recent session information is "sessionstore.js".  That probably now contains your lost-tabs state.

Copying "sessionstore-backups/upgrade-js-<the most recent YYYYMMDDHHMMSS that's there>" onto "sessionstore.js" should -- theoretically -- restore you to the exact state you were in before the upgrade.  Starting up Firefox again should bring back the pre-upgrade tabs.  (Or if you've somehow upgraded multiple times since the loss of session, the next-most-recent YYYYMMDDHHMMSS file.)

If that doesn't work, copying over "sessionstore.bak" or "sessionstore-backups/previous.js" might work.  But as far as I can tell, the upgrade.js-* is the one you want, that'll give the best chances of getting it all back.