Till Now, I had no problems, but today, 13 July 2019, this extension seems to have some problems with syncronisation of SOME tabs. Indeed, some of my tabs cannot be loaded and I cannot see what was in these tabs since they are empty now. Please correct these bugs, maybe due to a Firefox update. Thanks in advance. Eric from France.
UPDATE, 15 July 2019 : PROBLEM SOLVED, THANKS !

Supernützliches Addon.
HINWEIS: Wenn das Addon nicht funktioniert, wenn es nicht speichert:
Geht in die EInstellungen des Addons (über ADDON) und löscht alle Sitzungen. Danach funktioniert es wieder.

NOTE: If Addon not works or not Save Sessions:
Go to Settings of Addon (over Firefox ADDON Menu) and delete old Session Manger Sessions. After this, Addon works normaly

Спасибо, отлично работает.
Также спасибо за поддержку Tree style tabs.

Super flexible, a must-have.

Awesome!

After a year or so of not using this extension, I was very pleasantly surprised when I installed it again today. It seems a lot more stable than what I was used to.
As a Tree Style Tab user, I highly recommend this extension.

Edit: The latest update has a much clearer layout which can be opened in a separate tab. Very useful.

Thanks for pointing out the Firefox bug that prevents add-ons from running by incorrectly flagging them. I love the Tab Session Manager. I rock 4 windows with ~20 tabs on each at any given time. Thanks a mill!

What I wrote previously (ignore):
"Please get your add-on signed, it has been removed from Firefox's list of approved add-ons and thereby inactivated on my computer. I'd give 5 stars otherwise."