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.