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.

It would be perfect if it stores not only the state of tabs but their history too

Tab Session Manager is everything I expect from it. But:
I have chosen 'Restore previous session at startup'. On the other hand, I frequently use Firefox feature 'Send a tab from Firefox on Android to other device'. That means, I share and send many URLs from my Phone's Firefox to my Desktop's Firefox, while my PC is shutdown. Whenever I open Firefox on my desktop, the browser receives and opens those in new tabs.
Here's the catch: these shared tabs are opened at the startup, before Tab Session Manger restores my previous session and when it restores the session, it completely wipes the received shared URLs' tabs. If developers find a solution for this annoying behavior, it would be great.

Super flexible, a must-have.

Awesome!

Very good at saving tab states, even in disastrous scenarios. However, it does not play well with "Tiled Tab Groups". While Firefox's default session save function (in the rare scenarios in which it actually functions properly) will restore with the tab groups intact, Tab Session Manager will destroy the group states, and loads them all in a single group. I'd suggest then that it simply doesn't seem to respect hidden tab statuses? That said, still the best we've got in Quantum by a mile so far.

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.