This extension has got to the point that I no longer run Firefox ESR in order to use the old Session Manager. Sure, it still has some bugs and missing features but it is quite usable. If you have problems losing sessions try the Export menu item to make manual backups.
To all those who complain about this extension not being as full-featured and reliable as the old Session Manager, remember that sienori started from scratch with a Web extension API that was so unfriendly to session management that none of the old extensions were ported. I think he has achieved a lot and encourage him to continue improving Tab Session Manager. Thanks.