I tried spoofing Safari's and Chrome's User-Agent string in Firefox and business.apple.com still blocks me because I'm not using a real Safari or Chrome. So they appear to be using feature detection of some API that's not available in Firefox. I see that the site also blocks Apple's own Safari browser on iOS.
Looks like this site was been blocking Firefox users since at least 2018, according to the bug report on Mozilla's webcompat.com issue tracker:
UA spofing used to work some time ago, as I was definitely using it that way.
it seems they recently changed something, because now I can't even load the login page properly anymore due to X-Frame-Options disallowing the embed of the auth iframe.
Looks like this site was been blocking Firefox users since at least 2018, according to the bug report on Mozilla's webcompat.com issue tracker:
https://webcompat.com/issues/18964