In my experience Firefox handles a large amount of tabs very well, while Chrome tends to eat a lot more RAM in the same situation (while doing its best to hide that fact).
On the other hand there are some websites that will easily demand a gigabyte of RAM on their own, or that use excessive amounts of CPU time. Chrome handles those cases much better than Firefox. But those are outliers, and if I have to visit them I just open them in Chrome.
don't really think a bloated spa is ff's fault..
my standalone slack on macos takes ~500M
Every single performance complaint I've seen post Quantum release has been from a Mac user.
For comparison, I currently have two browser windows open and zero tabs. One is this comment box, the other is a GitHub page. Firefox is using a grand total of 3GB RAM, about 700MB of which is for the WebExtensions process so figure about 2.3GB for a grand total of two pages.
This is actually an improvement on the way things used to be, but if I quit the browser now (as I probably will soon regardless), these same two pages would take up only about 200MB, which is still a lot for what it is displaying and running, and the WebExtensions process would reset to about 200-300MB. I have Waterfox running in the background too, basically Firefox 56, which I use for a single website with exactly one extension taking up over a gig by itself with a single page open.
I'm not complaining, again, it's a fact of life that I've accepted and it doesn't hurt me any, but if I changed my habits I have doubts about how much I would actually use Firefox. Maybe I still would after considering my other options, but the web isn't as interesting as it used to be and I have other means of getting what I want off it now so I don't really require a web browser I can throw a million tabs at since tabs themselves are pretty harmful.
That said, if there are browsers that do a better job nowadays, and I stopped looking a long time ago, I'm not convinced they would do much better than Firefox currently does.
Second, it sounds like you have quite a few addons running that each hold quite a bit of data in memory (like ad blockers). Try safe mode and see if behavior changes (though ads may now swallow quite a bit of memory now).