Thanks for reaching out, and apologies for the delayed response here, we wanted to make sure there was not any updates on this topic.
Can I ask if the
security.webauth.u2f flag has been enabled in Firefox? You can find it by entering
about:config into the browser’s address bar. From there, you can search for
security.webauth.u2f and make sure it’s set to
true. Once that has been set, you should be able to add a new security key.
If you have any issues with that however, it may be worth setting up in Google Chrome first to configure a new key, then you would be able to sign-in with Firefox.
Sorry, I didn’t have a more definitive answer to provide here but we’re limited in our ability to troubleshoot issues on your browser, especially while official support is still not something we’re quite able to offer at this time.