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...
"Permissions tweaks for Linux"
The very last section of the README in the U2F Support Extensions repo is entitled "Permissions tweaks for Linux" and contains a link to a list of uDev rules supplied by Yubico that need to be added for Firefox to be allowed to access Yubikeys on unix systems.
The Where & How
The udev rules required can be copied from the Yubico/libu2f-host
/lib/udev/rules.d/70-u2f.rules on your local system. Once copied there the new rules can be
reloaded using the following command, which needs to be run as root:
udevadm control --reload-rules && udevadm trigger
Now once Firefox is restarted it should be able to talk to a Yubikey and pass the U2F support checker?