I think Firefox "weirdos" are probably under reported in general because they go to great lengths to hide themselves from analytics. Therefore I also think the number of "weirdos" who change their UA while still allowing JS analytics is basically nil.
On any consumer site, mobile UAs just overwhelm the desktop ones anyway. Nobody cares about Samsung Browser, why would they care about some shit that's way below the fold.
I think this is a self-reinforcing feedback loop because of bad analytics.
Maybe initially some big, popular, otherwise important sites only supported some Chrome.
Then the 'weirdos' discovered that it worked perfectly with only one mouseclick changing the user-agent.
Time passed, more and more 'weirdos' spoofed their UA by default, because clicking is boring.
Analytics saw less and less FF, more Chrome(simplified).
Management saw less and less need to support anything else.
Pseudo-Cyber-Security rose up high in the sky, shining so bright, that it seemed just right.
Analytics still spitting out falsehoods...
...damagement...
...repeat.