It works with Firefox 60, Google however sometimes relies on Chrome quirks not specified in the standard which may not make it usable in Firefox. (Last I checked)
It does not, and Google does not yet even attempt to use WebAuthn. They're still using the (now legacy) U2F authentication mechanism, which firefox does not fully support. It works in some sites if you enable it in about:config, but not all. Like google.com.
Hmm, yeah, I was more aiming at "Firefox has working WebAuthn in the newest Version, Google just doesn't use it properly or at all", I should have expressed that better.