Most of these chromium-based browsers are intended to address privacy concerns. Firefox (mostly) respects your privacy.
There are also sometimes compatibility issues with Firefox because web developers only test on chromium and webkit. Anyone opinionated enough to put up with that is just going to use Firefox.
Primarily probably yes, but I think for example Brave or Arc Browser teams also had ideas for their own browser features instead of "just" making a more degoogled Chromium. Helium as well, I suppose, otherwise what's the point?
That's my (admittedly extremely limited) understanding. The Mozilla/Firefox people de-prioritized making Gecko a separate linkable library years ago, and it's no longer a straightforward thing to do. Which is a real bummer.
