Hi, MySession seems more robust than Tab Session Manager (TSM uses indexDB; it's json got corrupted so I lost alll my sessions), but maybe you could consider implement this TSM good things:
-'save session on close' option [without using 'save each __minutes', *even when eg the PC unespectedly shuts down from energy failure*, amazingly it usually manages to recover the tabs.]
-it restores tabs according to the Firefox 'Container' each tabs is when saving the session, eg I can have a session with all/some 'Work' or 'Personal' Containers. [Maybe this can't be done in MySession since regular Bookmarks (only for now until new API?) can't save the 'Container' status?]
-it opens the session starting on the tab you were when saved [not necessarily on the 1st tab] eg while being in the last tab (eg 9 tabs, you were in tab #9, opening session starts in tab #9, not in #1).
-if you click to expand the list of tabs of a session, > click on any tab > only that tab opens in the window you are.
Edit: I guess I was wrong about the last point, so 1+star (only used the addon 1 time because from the proposals, at least I cannot forego the containers function.)
The reason for 3 is (using lazy Loading) eg I don't want always to load tab#1, but [in the example, it could be tabb5 or whatever] start in/only load tab9, then i after i close it, Tab8 loads, and so on, until i stop and close the window, without loading the 1st tabs. Without the function, before opening the session i would have to put FFx on 'work offline' before tab1 loads , then put FF 'onlne', then click tab9 etc. (or arrange the tabs in different order before saving the session.)