Firefox 57 U2F support
With version 57 of Firefox comes support for U2F multi-factor authentication (MFA). This allows owners of devices such as the Yubikey to perform a second layer of authentication to U2F-supporting services. After entering your normal username and password, you are prompted to touch the flashing button on the Yubikey and are then authenticated to that website. This is more secure than SMS-based MFA and potentially more so than time-based one-time codes, depending on how the seeds for them are stored.
To enable support U2F in Firefox (why isn't it enabled by default?!), enter this in the address bar:
You may need to accept a warning about being careful. Follow that advice :)
webauth.u2f and then double-click
security.webauth.u2f to change its Value from
Some sites, including Google, only seem to offer U2F as an MFA option when using Chrome or Chromium browser but will hopefully rectify this soon.