Many aeons ago I worked on webkit, and the first step for a great many site compatibility bugs is "does it work with a Firefox ua string".

Because it is always easiest to just check ie/new-ie and then useragent gate everything else, that is what happens. Which is why we keep getting sites requiring chrome (new ie) or ie (old ie) - it doesn't matter if the site is broken due to reliance on chrome behavior vs spec, what matters is devs coding to a single browser and considering any deviation to be a bug in anything else.