I am very much an advocate for 2 factor authentication tokens or keys such as a Yubikey, these devices can interface with web browsers through a Javascript API that web browsers expose or in the case of Firefox not expose.

There is a community made extension that fills this gap until the Firefox devs get the U2F JS API implemented in version 57 or 58 however I was never able to get it to work, no matter what I tried and no matter how many times I ran the test it just kept popping up with a message saying "Please plug in your U2F device".

Then I found the source code on GitHub and the last line in the README contained the key...