This was a nice piece of news, so of course I went to check the release schedule.
https://wiki.mozilla.org/Security/CryptoEngineering#Web_Auth... states that FF 56 or 57 should have native U2F support, and 57 or 58 would have it flipped on for everyone. Promising.
https://bugzilla.mozilla.org/show_bug.cgi?id=1065729 is still open, but has seen plenty of progress.
Even with current version (55), one would expect lot of the groundwork to be in place. And sure enough, searching for "u2f" in about:config brings up a nice triplet of security.webauth.u2f* keys.