Everyone has limited resources. As a for profit company, the focus has to be on your bottom line. How many resources should a company use for some obscure corner case when the user can make changes?
Of course accessibility is important - ie screen reader compatibility.
A typical testing matrix in the US would be
- Safari for iOS
- Chrome for desktop and Android
- maybe Safari for desktop or you just tell Mac users to use Chrome
- Firefox if you have the time. But if not, no big deal.
We are definitely not going to test for a highly customized Firefox on Linux running over a VPN.