Sorry for the confusion with the initial version of this reply. I misread the question and only saw stateful IPv6 and not stateful IPv6 DHCP server. The ASA fully supports stateful IPv6 firewalling in the sense that it tracks client sessions, but does not support a stateful IPv6 DHCP server internally. The internal IPv6 DHCP server only supports stateless DHCP, assigning DHCP options to clients that have already configured their addressing via SLAAC. If you really need stateful IPv6 DHCP, you should configure it on a separate server and use IPv6 DHCP relay where necessary.