At least back in the mid 2000s if you ever saw banners that said a site needed Firefox to run properly it was because people were on IE which didn't support new standard HTML features. Not because devs didn't test their JS outside of Chrome. If you can't code for more than one browser you shouldn't be a web developer imho. Unless you're doing Electron or something.
In an ideal world we'd have about 25% to Edge, 25% to Safari, 25% to Firefox and 25% to Chrome.
Firefox will never match Chrome head-to-head, and is a dying browser. Then only way Chrome doesn't win is if a new competitor appears.
I switched back to Firefox around version 57, and the only 'performance issue' was initially having to re-learn stuff that was muscle memory in Chrome. Once I got up to speed the user experience is indistinguishable from Chrome - even on the crappy old netbook I sometimes have to use.
The thing that might kill firefox is the aforementioned shitty web devs with their bogus UA checks, Firefox having a history of bad performance/stability, and most people already using Chrome with no reason to consider switching.
People who actually write websites people use and thusly they support everything from IE11 and up.
Or the ones that shit out the latest trend in webdev which only supports the Chrome version the author used.