I just lost my entire year's worth of valuable tabs. Where did they go and how do I retrieve them?

Did that happen after a restart, not restoring a previous session; or just while browsing, on its own? Anything special happening, like system updates or any maintenance going on?

Is that the only thing missing, the rest of the profile data like settings and history is still there?

Did any of the entries in History › Recently closed windows provide anything relevant?

No changes. I think the browser may have auto updated prior. I have been using the new vertical tabs bar (and love it) since it was introduced. I made lists for clients in separate tabs. After the update the entire menu is gone. Nothing else has been lost. I still have bookmarks, but the tabs section has been removed.

Can you try following How to restore a browsing session from backup to look for potential older versions of your session store files? Usually upgrade leaves a time–stamped version for later restore if need be.

Also for good measure see if any past closed windows are available in the History menu, in case it simply got closed: (each row represent a whole window, potentially full of multiple tabs)

I followed the above: All page visits in my history are there. It is only the tab containers that are just gone. I checked the system restore. There are no versions to reset. I adjusted Settings to Restore tabs from Previous session, just to be safe. But it looks like everything in my container tabs was wiped.

You can follow the steps here: support.mozilla.org/questions/1617462#answer-1861735 to try to see if there is any older copy in your profile folder set aside from earlier perhaps.