However, almost every web site or web app I make in my professional capacity gets tested with, and is expected to work with, ~95% of browsers. That means testing against Chrome, Firefox, Edge, Opera, UC Browser, Samsung Internet, and Internet Explorer. Edit: I forgot to add Safari, which I also test with.
Anything less is not my brand of professionalism, because I am convinced my clients expect me to leave my personal thoughts and convictions at the door. If you want to only create sites that work in Firefox and maybe not elsewhere, then you can do that - but doing it ethically means telling the client upfront that you won't accept work that requires the site work for the widest audience reasonably possible.
That said, there are two caveats. The first is ethics. I do not leave my ethics at the door and if I am being asked to do something unethical I will refund the money and walk. If it is illegal it will be reported, as otherwise I am an accessory. Clients agree to this upfront. The second is client requests. If the client wants it to work in Firefox and they tell me not to bother with the other websites, despite my advice against it, then that's what I do.