The demo at https://www.passkeys.io/ still doesn't quite work on FF 120.0b6: after clicking on "create a passkey" I get a pop-up instructing me to "Touch your security key to continue with www.passkeys.io.". I assume that this is referring to Touch I...
For the coders: all it takes is setting the NSTextContentType property of the NSTextContent object of the corresponding text field to NSTextContentTypeOneTimeCode, see here. I am not familiar with the Firefox codebase, but someone who knows their way...