Here is what you need to do to configure IPv6 with a fresh OpnSense Installation.
Step 1: Download and Install OpnSense – here are the USB instructions
Step 2: Access your new install at https://192.168.1.1 the default login is root and password is : opnsense
Step 3: Go through default config wizard, selecting DHCP for your WAN connection
Step 4: Select Interfaces > WAN > and set IPv6 Configuration Type to DHCPv6
Step 5: Check that LAN > IPv6 Configuration Type is now automatically set to Track Interface
Step 6: Restart your OpnSense box – you should now see an IPv6 address for both your WAN and LAN interface on the system dashboard.
Gateways should show WAN_DHCP with your IP and WAN_DHCP6 dynamic
These same basic instructions work for pFsense as well although the menu names may be different. My testing showed pFsense detected the IPv6 address automatically and setup everything without intervention on Webpass.