It's not FUD but there's usually more to it than just "Firefox". Usually has something to do with security plugins. There are sites that do not work properly with Ublock or Noscript installed, even when you turn them off for the site. I've experienced it many, many times. It happens to me most often ordering food, because a lot of local restaurants sites are janky as fuck, but I've also had issues with more well known sites. Southwest airlines has been problematic for a couple years now. My credit union also had issues with parts of their online banking app, but that thankfully got fixed after a year or two.

TL;DR - it's a real thing.