That’s the go to way to do it, you only allow the browsers that you know work, Firefox probably works fine for most things but maybe one feature breaks and instead of fixing it they decided to remove it from the whitelist

That dude ain’t joking. You can bypass similar bs by changing user agent

The server just sends files as responses to http requests. If the server is playing nice and just checks the user agent reported by the client, then that’s what you would expect.

And, it might make sense to do so in order to provide a product that meets certain requirements. It is certainly worrying that they need to do that, and not a good thing to make products exclusive to proprietary clients.