Right now firefox first establishes HTTP/2 connection, looks for Alt-Svc header for advertised HTTP/3 support and then attempts HTTP/3 connection. It would be nice, If there will be preference to specify domains with known HTTP/3 support, so firefox will try HTTP/3 connection with them first without need to establish HTTP/2.
Looks like network.http.http3.alt-svc-mapping-for-testing pref does exactly that, but it lacks wildcard support. I would like to have it working with wilcards to: *.google.com;h3=:443,*.googleapis.com;h3=:443,*.facebook.com;h3=:443,*.fbcdn.net;h3=:443 etc.
Forcing HTTP/3 connection first allows to bypass some restrictions from ISP.