Apparently I am unable to connect to an ipV6 test server when I connect through my Airport Extreme A1408. If I connect my MacBook Air directly to the Comcast modem via Thunderbird Ethernet Adapter, I can. So it appears something is amiss in the router configuration. The Internet tab on this router from AirPort utility shows a blank for the IPv6 address. It also shows blank on Preferences/Network/TCP-IP tab. I've been through the power cycle of both the modem and the router a number of times without success.
Since ipV6 seems to work properly when connected directly to the modem, I assume it is due to the Airport Extreme.