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

Works great but does NOT Support Multi-Account Container. Every time session is restored, you'll have to reopen the tabs in the desired Container which is really annoying. Otherwise, it's a great must-have add-on.

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.

Works great but the only issue I have is when i restore a session sometimes it will also restore my bookmarks and open them up in tabs so a session that was like 5 tabs when I restored it suddenly has over 100 tabs.

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.

Phenomenal!! Essential for my productivity. Cannot recommend highly enough