I was thinking about this recently too. I'd like to have a "session" per window that I can open and close and resume at any time. Instead of having to load all or nothing at browser startup.

I use firefox with A LOT of profiles, for every container area that i have in my life, with this I use some email alias to sync separate bookmarks and addons settings, you should try to...

Another way to save the pages is with multi tab group addon to reenable that old ff feature...

example: profile 1 personal, 2 work n 3 school, fix the tab about:profiles to quick acess to all or create ff shortcut for every profile... multi tab feature I use for some big research like: an ac eletronic board, a group with schematics links, and link to every component datasheet that im trying to fix, when im not fixing it, i go for another group and got it nice n easy