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."

From now Firefox works only with signed add-ons.
Please sign your add-on.

PS temporary solution from Firefox docs
about:config xpinstall.signatures.required = false

The best currently for Quantum.

Thank You So Much Dear "Sienori" , For This Brilliant addon , Excellent addon

excellent, the best session manager at the moment. if i may, i'd like to point out a little problem regarding favicons and (i think) "auto tab discard" extension: when TSM saves a session with some "discarded tabs" by that extension, the modified favicon disappear when reopening the session.

This is a must-have, life-saving add-on! Ever since TabMix Plus stopped working, I was looking for an add-on that would ensure that my tabs are restored when Firefox has a bad crash and can't restore itself.
But this add-on does so much more. It allows you to store your session manually or automatically. So whenever you notice that you are missing a tab (web page) from a few weeks ago, chances are good that you can find a backup.