The first row and first column should stay visible when you scroll with such a dense graph.

Not that brave after all.

I routinely have more than 100 tabs open. Firefox doesn’t crash for me or use much RAM. Many tabs is a normal use-case and Firefox working worse than alternatives in such scenarios is a failing of Firefox.

But again, mine handles it fine. So potentially OP should try again with an up to date Firefox.

Maybe I’m being stupid but doesn’t Firefox have tab-groups?