HTTP knows nothing like windows or tabs, just a user agent. And that's a good thing. Did you know there are several user agents that do NOT run in a windowed environment? I'm thinking of links, lynx, and Arachne. Also, most smartphones don't use windows.

So no, without very tricky hacks on the client side, you won't be able to have per-window sessions.

Perhaps private browsing mode may work, it should isolate the visited website from the other websites.

You could also create several Firefox profiles, and use several Firefox instances in parallel. You may need to add a command line parameter like "-no-remote" or something like that to prevent firefox reusing windows. (Or at least, it worked that way 10 years ago.)

Alexander