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.
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.