URL:
https://myaccount.google.com/signinoptions/two-step-verification

Browser/Version:
Firefox 57.0a1 build id 20170914220209
security.webauth.u2f set to true
security.webauth.webauthn_enable_usbtoken set to true
security.webauth.webauthn_enable_softtoken set to false

Operating System:
Linux

What seems to be the trouble?(Required)

• Desktop site instead of mobile site
• Mobile site is not usable
• Video doesn't play
• Layout is messed up
• Text is not visible
• Something else (Add details below)

Steps to Reproduce

1. Navigate to: https://myaccount.google.com/signinoptions/two-step-verification
2. Attempt to add a U2F device ("Security Key")

Expected Behavior:
Google let's me add a U2F device from Firefox

Actual Behavior:
Google informs me that only Chrome supports U2F and to change web browser

Screenshot