Joan, interesting idea that it's a firewall problem. You can disable the firewall by logging on to the gateway as YSR50 said. There's a menu on the left that says Firewall. "Disable entire firewall" is an option in "Custom Security." I also agree with YSR50 that disabling the firewall is generally a bad idea. But if this works, it's pinpointing the problem. I don't know if it's necessary to reboot the modem after changing these settings. Sad that we have to do Xfinity's work for them. Let us know if this works for you.
Edited to say that if you are willing to be the test case, please tryto disable the firewall on ipv4 as well as ipv6. By clicking on the i info button on my wifi, I can see that some of my devices are running on ipv4 and others ipv6. Thank you for your service. :)
Also, I finally noticed last night that my apps weren't updating automatically on my ipads, but were on my phone. I was able to manually update some via wifi, but others hung up, and I had to plug into ethernet to finish the job.