Yeah, it seems like there's always this split on every Hacker News thread about Firefox.
Personally, I've use Firefox as my primary browser since before I started using Macs, and after 6 different Macs between home and work, I've never had any problems with it.
I want to love it, but even after a fresh install of MacOS every time I go to use it on my 2015 MBP it grinds to a halt.
I'm sticking to ungoogled-chromium for now. I really hope it's resolved down the line!