I run Arch Linux on this exact hardware and Firefox runs amazingly well there. It only runs like crap under macOS, which makes me think there's something with macOS specifically that's going on there.
Such a two week period has just ended, and while FF does feel snappier than before, I'm running into exactly the issues you describe. When using Firefox, my four-year old MacBook Air pretty regularly slows down to the point where my music starts stuttering and even force-closing FF takes a while. I've had bluetooth devices disconnect too.
Now if it was just that, I could perhaps accept that maybe my laptop just can't handle my browsing behavior anymore. Unfortunately the same thing happens on my 16GB RAM Mac Mini. Not as often, but still.
I don't have these problems with Chrome, even when I clearly have too many tabs open for my memory to handle. Occasionally my music will stutter when I open multiple gmail tabs in Safari (I use Safari for stuff like email and banking), but it's rare, and nothing like what happens with Firefox.
Thing is, it's not even that I have so many tabs open. It's always been fewer than Chrome, and with fewer active extensions to boot. Furthermore, it also happens on my other computer. The other one has 16 Gb of RAM, so it can handle quite a bit more, but the problem happens often enough that I also switched back to Chrome there.
At work, I have several GUIs in browsers that I access through Chrome, and they cause instant 3+ GB of usage. I can't test FF due to group policy, but anyway, Chrome can be a hog, too.
Are you saying that those same half dozen apps, when pinned in Chrome, do not cause any RAM issues for you?