[reaperhulk]

You need to enable it in Develop->Experimental Features, but also you need to be sure you have a CTAP2 key. "U2F" keys are all CTAP1, which is not supported yet. Yubi's 5 series are CTAP2, as are the blue keys with a 2 printed on them.

[mediocrejoker]

Cool I just tried it with Fastmail and a Yubikey 5 and it worked. Gmail however still checks if you're running chrome so it doesn't work.