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 !

Hi, I had an indexDB failure [after updating to FF 68], tried the Github 'Internal UUID copy-paste folder trick', didn't work; seems TSM's own json got corrupted, so I lost alll my sessions.
Please would you consider using eg Bookmarks [like MySessions] instead of indexDB that is so prone to corruptions?
Other than that, great addon, thanks.

Dobry dodatek. Brakuje tylko jakiejś synchronizacji między komputerami. Wtedy niewątpliwie 5.

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.