Firefox has been, and still is, my primary browser since before Chrome even existed so, definitely not FUD. Also, it’s generally not Firefox’s fault either, but instead the developers of websites that don’t work in Firefox are usually doing something that isn’t standards compliant.
First to come to mind is that I can’t log into the account management part of the pet boarding company I use when in Firefox. Another scenario is that a lot of movie streaming sites won’t give Firefox video higher than 720p so in that case, Edge is often the only browser that can receive 1080p video. From my understanding the movie studios are the ones to blame for this.