I'm one of those users. Actually I think I abandoned Firefox last year.
I want to like Firefox. But every single update for several years has reduced it's utility. Development is no longer focused on the user. Who knows what it's focused on at this point, they say the user, but looking at the design decisions it is easy to see that that isn't true. I don't like being bullshitted.
I think generally the idea of "web browsers" is a negative thing. The internet is supposed to be protocols, the web is supposed to be documents. Having a window that you open that runs more stuff than your operating system unless you're writing code is strange, it is a UX entirely driven by massive technical debt as a result of bad decisions piling on top of one another for decades.
These days as far as browsers go, I use ungoogled chromium, bromite, and gnome web. I've tried pale moon and a few others and they're generally not what they're pitched as. It's hard to build a browser from scratch because of what I mentioned above, and I get that, and the main problem with web browsers is that the web is broken, but I live in a world where I need one and Firefox doesn't deliver.