Of the many tab managers I tested, Tab Session Manager is the one I love and use most. I has all the functionality I need implemented in a lean, efficent and no-frills way. Easy to understand, trivial to learn and has never failed me (using it about a year now).

It comes with built in manual sync to your own google drive account (no 3-rd party servers or sign-ups in between Firefox and your G-drive account).

It auto-backups all your Firefox windows & tabs automatically locally (time interval can be set in user settings) and is the best browser crash-insurance to date.

Tab Session Manager is a well designed, cross-browser and efficient FLOSS extension: Free (as in free beer), Libre (as in liberte, egalite, fraternite) and Open Source Software, with code access on github. It is well documented and in continuous feature development and bug maintenance (due to the ever evolving browser API changes).

My warm gratitude to the main developer @sienori on github.com

How about add features:
> Add to current tab to session group.
> Override session group with current window tabs.
> Always show searchbar.
> Order group by drag.
> Provide recycling bins.

saved my a*s off

Great add-on, just what I needed. Many thanks!

>>> "IndexedDB" ERROR MESSAGE ON NEW INSTALL!!! RESTART THE BROWSER TO FIX!!! <<<
#. IMPRESSIVE SESSION MANAGER - AFTER RE-OPENING, HAD ALREADY A SAVED SESSION (NOT CURRENT) FROM 8 HOURS AGO WHEN I PREVIOUSLY OPENED BROWSER!
VERY COOL!!! :D ;) ;D :P :B

Greate addon. Let's add "recomended" badget?

Génial ! A tout récupéré après un vilain crash.

I use it approx 3 years! The Best!

WARNING: this addon is not compatible with «Simple Tab Groups» and would pull all grouped tabs into one window. Luckily, STG has a backup function.

Firefox can be garbage with saving your sessions. Can't count how many times it has failed for no damn reason, and this add-on has saved me countless headache. Definitely a must have

Add-on works without issues. The user experience, however, is not exciting at all.
Session management bar should definitively be on the right site, so you can access tabs by moving the mouse just down, not across a half screen.
Design is disruptive compared to the beautiful Firefox.
And this shouting icon that only means that sessions are not synced (which is every time you change a tab) is absolutely frightful. It is better not to even enable sync.
Highly recommend to hire a UX designer...