Here's a bug, if you disable 2FA and use Passkeys, then this works fine on desktop, but if you the go to mobile via the phone web browser (also Firefox though not relevant) then you can add a passkey for that device and it saves fine and shows in the list of passkeys, but when you go to log in via mobile web browser it tries to use Google to find the passkey which obviously doesn't exist
Not really a bug, its expected behavior as you would need a portable passkey app, Proton pass for example.