it seems I entirely missed the red text on Firefox that says "Your browser does not implement Conditional UI" so what Firefox did was to probably prompt for the old WebAuthn and I guess that's Windows Hello which makes a lot of sense to me. I wonder whether Firefix will also show me the QR code signin as the default on desktop once they implement it. Like you said...in my opinion this makes no sense, obviously as a desktop user using a desktop site I want to remain on my desktop hardware device.

Or am I misunderstanding you and you want to log in on desktop with the same passkey that you generated on mobile? I think that is never supposed to work, it's supposed to be tied to the device.