[Freak_NL]

Facebook engineers responded to this in the U2F issue on the Firefox issue tracker. It turns out that for this particular technology determining whether or not your browser supports U2F is not yet feasible, so they resort to user agent sniffing. You can use Firefox with the third party plugin if you spoof the user agent to Chrome.

[fulafel]

The comment said they could detect it but they are doing user agent sniffing because it's how their code already worked.

"Doing feature-detection of a U2F API instead of User Agent sniffing would have required additional engineering effort due to how our system already works" (https://bugzilla.mozilla.org/show_bug.cgi?id=1065729#c264)