That's good to know, I don't know how well it would work though I feel like I enabled something about closing background tabs to reduce memory load (it might have been what you said, it might have been something else I don't really remember) and it helped a little bit but it still ended up chewing up a lot of memory.

Setting the limit though did help immediately. And stop the overconsumption problems, occasionally a couple of tabs crash here and there but it doesn't freeze or worse cause other apps to slow down and freeze. Which did happen before.

Something I didn't consider when answering earlier is that even if Firefox did have good RAM usage limiting built-in I probably still wouldn't use it or recommend it, because one of Firefox's biggest problems is that it leaks. And memory leaks will not be negated by Firefox's built-in RAM limiter but they will be by systemd's (or anything else you might be using instead) Firefox would still crash in the event of a leak but it's still better than it taking gnome or other apps with it, or freezing your system entirely.