Both Mozilla and Google are aware of the issue. That's all I can say at this point
No ETA on a fix.
Interestingly enough, that Mozilla page lumps Google and Facebook into the same issue (not related to the user agent), but changing the user agent to Chrome actually allows Facebook and U2F to work in Firefox Quantum (I've confirmed both registering a U2F device and logging in with a U2F device).
The U2F demo page does work without changing the user agent (demo.yubico.com/u2f) - I haven't tried the other various U2F demo sites out there, however.