pam_u2f - is an important library because it supports many inexpensive u2f keys available from different manufacturers. You can log in as a user or log in via ssh, for example.
Luckily it works fine on FreeBSD 13.0 (pam_u2f-1.2.0, u2f-devd-1.1.10_6, libfido2-1.8.0).
Here is the Makefile...
I've installed the latest Firefox package (firefox-61.0.1_1,1) on my FreeBSD 11.2-RELEASE, and activated u2f (security.webauth.u2f -> true), but the yubikey is not activated/recognized by Firefox.