[tialaramex]

Ah, for sites using U2F rather than WebAuthn you may (depending on Firefox version) need to explicitly tell Firefox to emulate the old U2F APIs.

https://support.yubico.com/support/solutions/articles/150000...

https://blog.mozilla.org/security/2019/04/04/shipping-fido-u...

My GitHub has two Security Keys registered via Firefox this way.