For a regular web page such as slashdot.org, Firefox needs to resolve more than 40 domain names. Each domain name is resolved twice, once for an IPv4 address and once for an IPv6 address. This results in lots of DNS requests, slowing down your web access. If you are like 99.999% of the population without IPv6 access, translating domain names in IPv6 addresses is useless.
To disable this functionality, type about:config into the (firefox) address bar.