@waterfire I am also a long time Firefox user who has switched to Vivaldi.
There is nothing better than Session Manager by Morac for Firefox. It automatically makes session backups, and restores them superbly.
As far as I know, Vivaldi only has a manual session backup, so that is useless for me.
Session Buddy is what I use. It is nowhere near as good as Session Manager by Morac for Firefox, but at least it makes automatic session backups.