The likely reason why they don't support Firefox is because it has less than 5% marketshare. It isn't a Chrome only site. You said that they support Chrome, IE, Edge (if they support IE then I'd assume that they might also support pre-Chromium Edge), and Safari.

That is just the nature of using a niche platform. I primarily use Linux on the desktop. I have to keep a Windows install around for the times that I need to do something that can't be done on Linux. Resources are limited and so high marketshare platforms are prioritized. That's just how it is.