Haven't tested yet, but finally found an extension that's almost replacement for the official Mozilla extension Session Manager, which didn't get a version for new extensions when legacy extension support was removed.

Only thing I'd love to have is the option to replace Firefox session restore with this so that when starting Firefox, it would open Tab Session Manager as popup - like Session Manager. You could there pick a session, AND unselect tabs if you wanted to leave some of them out. It also gave option to just start fresh, but of course you could load a session later. This is what I'd love to get when the browser starts, no loading previous session at startup but instead asking for one before opening browser windows (or maybe opening one empty browser window, then either loading selected session on that and new windows (if the session is multi-window).

Also LOVE that it supports Tree Style Tabs!!!