Well, I figured out the issue, so thought I should post what I found, even though I feel a bit stupid now. Seems the main problem was a lack of knowledge on my part and that of Comcast Tier 1 support. Basically I had my gateway IP and static IP reversed.
Turns out that since we were originally using the Comcast CM as a modem/gateway without a firewall behind it, and then later set up the firewall in the CM's DMZ, the gateway IP was functioning as our public static IP. It didn't help that the person who set up the network had documented the gateway IP as our static and vice versa. And Tier 1 support apparently had no clue. It took Tier 2 support to point out my mistake and of course it seems fairly obvious to me now. I suppose my one remaining question is whether this is typical behavior of static IP implementations or specific to Comcast and/or other ISPs? Either way, lesson learned.
I should note one thing. I am 99% sure I did try reversing the gateway/public IPs when I first failed in configuring the static WAN interface, and that it did not work. What I did differently this time, however, was power cycle both the CM and FW, as opposed to just rebooting each; a simple step, mentioned by others in various posts, that might have helped me solve this sooner. Another lesson learned. 🙂