Seeing this image right now.
Background info from Firefox Troubleshooting Information:
Build ID 2019022821563
OS Linux 5.0.2-arch1-1-ARCH
This is not resolved.
Right now if I get a login prompt from GitHub while I'm using Firefox, it asks me to enter my password. If I change my user agent to Chrome, still using Firefox, it properly adds that I can press a button on my security key to log in.
So U2F is still being blocked in Firefox based purely on user agent.