If you are using Firefox with multiple tabs open and you are experiencing high RAM usage you can fix the issue with setting this tweak in about:config to TRUE (disabled by default):
browser.tabs.unloadOnLowMemory -> true
based on this thread and the linked bugzilla discussion (where they identified the issue but haven't rolled it out on main yet) it sounds like this feature isn't currently working as intended
https://old.reddit.com/r/firefox/comments/co6xng/browsertabsunloadonlowmemory_how_much_is_low/
@nikolal yeah, it sounds like the memory detection algorithm was busted so it acts inappropriately in some scenarios (until the fix ships)