Although the form result made it sound like a macOS-only issue, I actually have observed (and continue to observe) it on both macOS and Fedora.
EDIT: In the thread, am seeing the reference to how Firefox-on-QUIC works if one has IPv6. My ISP (Frontier FiOS) infamously doesn't support IPv6, so I'm out of luck there where Firefox is concerned.
https://bugzilla.mozilla.org/show_bug.cgi?id=1979683#c3