Mozilla for Windows has IP6 support
The Internet Protocol Version 6 Mozilla bug 175340 has been fixed. This means that Mozilla on Windows now supports IPv6.
If you're on Windows 2000/XP/2003 you can activate IPv6 by opening a command prompt and type
ipv6 install. Remove it by typing
To get IPv6 working you need to install the Advanced Networking Pack
Read more about the IPv6 support on Windows at Microsoft site.