Unfortunately it's not up to Firefox to determine which Browser they want user to use to access their site. How and why sites choose what they do isn't something Firefox can control here. Until their site security permits that the limit of this.

But the error isn't that my browser isn't supported, it's that the browser doesn't have a certain feature - which it does, because I've used it before. It seems like Firefox should know what can block it.

According to the answer here, Firefox should support keys : https://support.mozilla.org/en-US/questions/1240881

Did you contact site for the 2FA about this login issue with Firefox?

Yes, i contacted Apple but they said they can't help and should talk to Firefox. What can I do here?

I understand your frustration with the issue you're facing while using physical Titan security keys for 2FA on iCloud with Firefox. It's possible that there might be some compatibility issues specific to iCloud's security requirements. Even though Firefox generally supports security keys, some websites may have specific configurations that affect their functionality. As a first step, you can try clearing your browser cache and cookies, and then attempt to sign in again. If the issue persists, you can also try disabling any add-ons or extensions that might interfere with the security key functionality. If the problem still isn't resolved, reaching out to iCloud's support team might be helpful, as they may have more insight into any potential limitations with using security keys on their platform. They might also provide guidance on alternative methods of setting up 2FA for your account. I hope this helps, and if you have any further questions, please feel free to ask.