The problem is that there's a buggy u2f library out in the wild distributed by Google that depends on Chrome-specific impl details. This means that while the pref works for sites like Github, it doesn't work for Duo or Google logins or anything that uses u2f.js. Sometimes it doesn't work to the extent that these sites break in other ways. There's nothing that Firefox can do about this; aside from waiting for it to be fixed.

Ultimately the thing that is going to be shipped is probably webauthn.

I think it is effectively part of/superseded by Web Authentication, which is mentioned and links to:

https://groups.google.com/forum/#!msg/mozilla.dev.platform/t...

First sentence in that thread is "Web Authentication is backward compatible with FIDO U2F second-factor tokens, and also supports more advanced capabilities in future FIDO 2.0 devices."

Uhmm while not on the roadmap, according to their wiki[1] they are still working on it.

[1]https://wiki.mozilla.org/Security/CryptoEngineering