I only ever use Firefox, at work, at home, mobile. Since forever, as long as there has been a Firefox (and Mozilla before that, Netscape prior).
There was one time about 8-10 years ago that I encountered a site that didn't work in Firefox, was some weird proprietary training module at work.
So yes, I did encounter one site that didn't work in Firefox in the last ~20 years. One.
I think these people are real and predate AI. I think it comes down to a fundamental psychology in how the two browsers are marketed online. Firefox users would say "Firefox is better than Chrome." Chrome users would say "Chrome is good." Firefox is touted as a better browser on the principle of the freedom it gives its users, and paradoxically, that draws heavy scrutiny.
I don't think I am doing a Goomba fallacy here. People that claim to care about privacy and browser freedom will express disgust with Mozilla then, in the same breath, say they are opting into Chrome out of spite, which is 100x worse.
That is what is frustrating, indeed.
Firefox is not perfect. Nothing is perfect. There have been changes in Firefox over the years that I don't like. (I'm still hurting from it moving the tabs to the top, so annoying.)
In a scale of 1 (100% user-hostile) to 10 (absolutely perfect user-centered browser), Firefox only scores a, let's say, eight.
But chrome scores a solid 2, so obviously I'm not going to use it, ever, no matter if Firefox makes the occasional mis-step.
Weird!
Who do you think most developers work for?
Google, Facebook, two massive developer employers which are advertising companies.
There is a reason the quote “The best minds of my generation are thinking about how to make people click ads" is famous.
Because so many developers are doing nothing but that, pushing advertising.