Sometime some domains Firefox needs to resolve each domain name, one for an IPv4 address and once for an IPv6 address.
This results in lots of DNS requests, slowing down your web access.
you are like 99.999% of the population without IPv6 access, translating
domain names in IPv6 addresses is useless.
Try disabling this functionality, by typing about:config into the address bar.
Type ipv6 into the search bar and toggle network.dns.disableIPv6 to true.