In older FireFox(FF as follows) than 57 (ver <=56), "Session Manager"(S-M as follows) did a great job.
But in FF ver 57, S-M ended its life, due to the FF's change, cut off "XUL", which used in S-M, and moved to "Web extension".
This TSM seems an good "ALTERNATIVE" of S-M "JUST" for *FF 57*, for me.
TSM is *NOT* a successor of S-M. Different developer makes TSM than S-M. Words of "Session Manger" in TSM made me confusing, for the 1st time. If its name was "TAB MANAGER", might not be confusing for me.
You don't have to take risk to move from S-M to TSM, if you are still using older FF(ver <=56) or PaleMoon, which still has XUL, which is cut off in FF57. (I still use S-M on PaleMoon)
***
Following issue was fixed in TSM 1.6.1, using "tab lazy load" option. (FF ver 57.0.1 x64)
Good job! (2star -> 4star)
***
at TSM ver: 1.5.0, FFver: 57.0 x64.
"Open" with 100-200 tabs with 4 windows saved, only 1 window opens and 15-20 tabs only resotred.
With or without Tree-Style-Tab. Even in case of tabs with only http/https-style, this occures.
Exporting sessions from this extension, 4 windows and 100-200 tabs seems all saved in the exported file, instead.
***