Recently I had an issue where a site was restricting itself to a single IPv4 address, unfortunately the server that hosted it also supported IPv6, but the IPv6 on the server was misconfigured. This caused Firefox to error out if it used IPv6 or work if it used IPv4.
I needed to disable my IPv6 browsing so I used icanhazip.com to check to see what my IP address was, and it reported an IPv6 address.
I then entered
about:config
in the address bar and searched for:
network.dns.disableIPv6
I set this to true, then checked icanhazip.com again, and got the IPv4 address. Now the site worked for my server.