U2F and app-specific passwords are great advances in FM login security.
A post comparing U2F with the standard Yubikey OTP:
http://www.emaildiscussions.com/show...7&postcount=24
Chrome supports U2F, Firefox does not.
Sadly, Mozilla has yet to implement U2F support.
Others might list other browsers that support U2F.
Amazon links for U2F capable keys:
Yubikey U2F only 18 USD
Yubikey 4 40 USD
Yubikey NEO 50 USD