See my PPS at the bottom of this please!

On the right way, but miles to go yet.

First, the author deserves kudos for investing his/her time to create a replacement for that long term friend of all of us who come here to devour the passing away of the classical session manager.

Importing old sessions is the most important and prominent function these days immediately after the release of FF57.

The makers at Firefox / mozilla may have underestimated the part that independant addon-creators have played over the years for the users - and the appreciation that users spent for those little marvels that made the base structure so popular.

Now, MySessions seems to have some labours to fight with yet. It is much better that the new Session Manager that made my i7, equipped with 12 GB of RAM, stall when trying to reconstruct a 6 window / 960 tabs session. It seems to try that all in RAM , the first time I saw a RAM usage of 96% on that machine - with an endless usage of virtual memory, blocking any normal use of the machine.

Now, MySessions seems to do better. But it also seems to try and load every single content of any tab - that should be optionally deselectable! In my case, about 80 YouTube videos all started to play, with the previously set AutoPlay also disregarded (not sure if MySessions is responsible for that). Plus old tabs that triggered some downloads reappeared too (all contained in the sessions, but if they would not have been re-loaded, they would have rested in peace there... Bad habit on my side, ok, but...

Opening the reconstructed sessions in new windows makes sense, but be sure to know what was your previously opened set of windows...

I offer and volunteer to act as a beta tester for future versions, as a heavy-duty user who always finds something... :)

So the two vacant stars offer space for perfection. Your turn!

P.S.: After posting this, I read the previous exchange between you and other users here. I understand that the question of having all tabs open is a known issue and subject to more work and a possible solution.

Would it be possible to freeze them all to hibernization piece by piece immediately at the time of loading them? Maybe optionally selectable in an options switch array? Who ever booted a DEC PDP11 or an MITS Altair 8800 or an IMSAI 8080, for that matter, some 35 years ago, loves those switches... ;)

PPS: Sitting at the machine with the previously 6 windows / 980 tabs in the continuous session. I can't find the previous sessions. I looked in C:\Users\\AppData\Roaming\Mozilla\Firefox\Profiles\phzsk6df.default\sessionstore-backups, there are a few files but none is called something like sessionsomething.

Before I found MySessions, I tried the other suggested replacement and had to stop that after several hours (!!!!) of activities by using task manager.
Can it be that "the other one" deleted all old session files? I had set the queue to a pretty large number, and there had always been long lists of previously saved session sets.
I understand this is not a help forum - where to turn best with such questions? Thanks!

Since there is no way to reply to a reply here, I try this way: (2017-Nov20, 01 p.m., CET)

The addon you suggested - thanks for that - has not got the most benevolent comments and was updated last time 2 months ago. What benefit should that have? Specially for tabs, that are completely reloaded, like a first load, during the reconstruction of the old lists? Wouldn't they get into the hibernation state only on the second start of the session they are contained in?