Let me explain: I've used Firefox since the beginning (and Navigator before that). I like Mozilla and I agree that most of the changes that they've done internally in quantum were kind of necessary (although many could have been handled better).
However, along the way, they've broken the way many people use Firefox and the only answer the frustrated users seem to get is "but it's fast".
They've changed the UI to be similar to Chrome, to the point where the reload button is not where it used to be. But it's fast.
They've broken the extensibility, which was (at least for me) the main reason to use Firefox instead of Chrome, Chromium or any of the others. You could do things in Firefox that the others couldn't even dream of. But it's fast.
While doing this, they've alienated plenty of plugin developers, people who loved the Firefox ecosystem, some of which have had to rewrite their plugins thrice already (with some being told to, essentially, "bugger off"). But it's fast.
As far as I can tell, it's using more resources in MacOS than the old version, at least the processor runs hotter and the fans kick in more often. I've had issues with the CPU use sky-rocketing, but I believe that has more to do with opening multiple tabs quickly rather than with one specific page. But it's fast.
Honestly, I haven't noticed any major changes in speed in Quantum (it's a bit snappier, that's it) and was happy enough with it's previous performance. IMHO what we've lost is not worth what we've gained.
I'd move to other browser, but which one? None of the main ones have the things I liked about Firefox and I don't really see most of the forks being able to keep up the pace. I guess that I'll stay, hoping that the things I love about Firefox will eventually come back.