1. System > General Setup > Enable IPv6 Support
   
2. Interfaces > WAN > IPv6 Configuration > IPv6 Mode: DHCP
   
3. Interfaces > LAN > IPv6 Mode: DHCP-PD (DHCP Prefix Delegation)
4. Interfaces > LAN > IPv6 Prefix Delegation: 1 / 64
5. Interfaces > LAN > IPv6 RA: Send IPv6 router advertisements, Flags = “Managed”
   
6. Firewall > IPv6 Rules > WAN > Allow IPv6 ICMP from ANY to ANY
   
7. Firewall > IPv6 Rules > LAN > Allow IPv6 from LAN net to ANY
   
8. Services > DHCP Server:

   To work around a bug in M0n0wall 1.8.1 (per “Maiku” on the idnetters forum http://www.idnetters.co.uk/forums/index.php/topic,27563.msg654698.html):

   1. Enable IPv6 DHCP server on LAN interface with any given IPv6 range.
   2. Reboot.
   3. Disable IPv6 DHCP server on LAN interface with any given IPv6 range.
   4. Reboot.
   

Here are the threads that helped me the most:

• http://pivotallabs.com/configuring-freebsd-9-1-as-an-ipv6-dhcp-client/
• http://forums.comcast.com/t5/Basic-Internet-Connectivity-And/IPv6-Troubleshooting-Comcast-IPv6-Start-Here/td-
• http://test-ipv6.com/