The built-in session restore feature only is one session deep: it's just the immediately previous session.

You might have been using the Session Manager extension: https://addons.mozilla.org/firefox/addon/session-manager/

Does that look like what you're missing?

By the way, did you use the Refresh feature? If so, your older sessions might be saved in the Old Firefox Data folder on your desktop.

Up until last year it would be several sessions deep and I had not installed session manager. Did not use Refresh. Added session-manager for future. Thanks.

Or maybe: the Tab Mix Plus extension used to have a session manager feature. However, it might have been removed/hidden/turned off at some point.