In fact Apple doesn't even bother supporting Firefox on some sites. Case in point https://business.apple.com
It was blocked in Chrome too until I toggled desktop mode on.
What are they doing, blocking based on the user agent string? How mediocre. They're clearly not doing feature detection.
On desktop it doesn't even list Firefox as an option at all.. It's pretty annoying for me because I use that portal a lot for work and I use FF on everything. Safari is not an option for me because I use Mac, FreeBSD, Windows, Linux, Android all mixed and I need the sync.
And yeah I think they're just blocking the agent. I haven't tried bypassing it. I kinda was afraid something would go wrong (I don't want to wipe 10.000 iPhones from the portal instead of 1 - lol)