Agreed, I can't understand where these experiences come from.

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 genuinely wouldn't be surprised if it was Google astroturfing every single thread relating to FF. Yes, Mozilla isn't perfect and had some questionable decisions, but we're somehow okay with the worse option out of the two? Somehow, Mozilla's decisions are enough to never use FF, but Google's infinitely many horrible ones are acceptable for.... What reason?