Unfortunately at the moment (2015-11-03), U2F protocol is still not officially supported in Mozilla Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1065729
U2F protocol is mainly promoted by Yubico company (who is selling Yubikeys)
Finally, Firefox 57 (called Quantum) supports U2F protocol but for the moment (2017-12-17) it’s not enabled by default. Here is how to enable it:
- Type about:config into the Firefox browser.
- Search for “u2f”.
- Double click on security.webauth.u2f to enable U2F support (the value should be set to “true“)
Source: https://www.yubico.com/2017/11/how-to-navigate-fido-u2f-in-firefox-quantum/