Looking at the code in the DNS service in Firefox, it kinda looks like this for localhost: 1) If IPv6 disabled in prefs or caller explicitly requested IPv4 onl

Looking at the code in the DNS service in Firefox, it kinda looks like this for localhost: 1) If IPv6 disabled in prefs or caller explicitly requested IPv4 only, 127.0.0.1. 2) If localhost explicitly listed in the IPv4-only domains list in prefs, 127.0.0.1. 3) If caller explicitly requested IPv6 only, ::1. 4) Otherwise both 127.0.0.1 and ::1. See nsHostResolver::InitLoopbackRecord in https://searchfox.org/mozilla-central/source/netwerk/dns/nsHostResolver.cpp https://searchfox.org/mozilla-central/source/netwerk/dns/nsH... and nsDNSService::GetAFForLookup in https://searchfox.org/mozilla-central/source/netwerk/dns/nsDNSService2.cpp https://searchfox.org/mozilla-central/source/netwerk/dns/nsD... for the key.af calculation that goes into that, as far as I can tell.