You can try to use the Time Machine to restore the places.sqlite file from just before the lost.

• https://support.mozilla.org/kb/Recovering+important+data+from+an+old+profile

That got me closer, thank you. It restored the history but not all the open windows in the previous session. Restore Previous Session is still gray. I tried Recently Closed Tabs and it only gave me the ones since the upgrade. Open to any other suggestions ...

You need the sessionstore.js file to restore tabs and windows.

• http://kb.mozillazine.org/sessionstore.js

• http://kb.mozillazine.org/Session_Restore

I had no luck restoring the Previous Session tabs after replacing the sessionstore.js file or going through the about:sessionrestore procedure. If you've got any other suggestions I'd love to try them.

Thanks!