I am struggling with snap and flatpak at the moment.
Today, I couldn’t get screensharing to work in flatpak-installed Firefox.
So I switched to snap-installed Firefox, and found I couldn’t use Webauthn there.
Fortunately, there was a simple fix:
snap connect firefox:u2f-devices
(If it is desirable to have more people using Webauthn, for security purposes, I’d have thought that it would be better if this was enabled by default?)