By default IPv6 is disabled on Firefox 4, to enable it:
- Open Firefox
- In the address bar, type "about:config" and press Enter
- Click on "I'll be careful, I promise. "
- In the search bar, type "ipv6" to locate the following entry "network.dns.disableIPv6".
- Set it's value to "True"
- Restart Firefox for changes to take effect.