Responding for visibility as this solution is popular but a bit unsafe, esp. with people removing the firewall rule and allowing any GUA DHCPv6. You can simply allow GUA source with LLA destination + add a blind default route: http://forum.mikrotik.com/t/xfinity-comcast-dhcpv6-configuration-change/156031/10
This works properly on both “old” and “new” “10G” networks.