firefox自体のセッションストアが飛んだのでプロファイルを初期化してこのアドオンから復旧しようとしたのですが、2019/3/25以降にバックアップされたセッションをインポートしようとすると読み込みには成功するもののセッションリストが「全て[0]」のままです。
なお、その間タブの数は1500～1600ほどで大きくは変わらないのですが、3/12に56MBだったバックアップファイルがどんどん肥大化、読み込める一番新しいもので258MB、今月には503MBになりました。

Пожалуй самое удобное и хорошо зарекомендовавшее себя расширение, пользуюсь больше года и не подводило до недавнего времени, когда количество вкладок перевалило за 400!! Теперь программа не открывает окно с выбором сохранений, сбрасывал настройки, удалял и ставил по новой.. не работает ((

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

It worked really well until the fateful day where all my sessions went missing, and I can't even use it anymore. I can't export my sessions either.

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.

It is a great add-on. But if you also use containers and you will restore a session all tabs are moved to the default container. I had 15 containers and aprox. 350 tabs. It takes hours to move each tab back to the container (I did not found a add-on for "mass-moving" tabs to containers). If you do not use containers: It is the right add-on for tab session. For containers + sessions search for 'session boss'.

Found out about Session Boss, a similar but far more sophisticated session saving add-on, and never looked back. Sorry sienori. Perhaps you two could join forces and make one that is the absolute best? Session Boss could use some more config options that this Session Manager offers.

The best currently for Quantum.