firefox never retries with ipv6 if it has failed once
If a website is dual-stacked, Firefox will first try to connect over IPv6 and fallback to IPv4 if it fails. However, Firefox never tries again to use IPv6 if it as fails once. Sometimes this leads to connection error if the connection is intermittent (so IPv6 will fails at the time of the request, but it’s not specific to IPv6, and Firefox will try over IPv4) and if IPv4 is broken at all (e.g. because IPv6 works so nobody notices it). So when the connection comes back, Firefox will be stuck to IPv4 and the site won’t display even if works with IPv6. Even Ctrl-Shift-R doesn’t clear the connection cache.