I've found there's a little more to this...
It appears this is also related to IPv6. I happen to be using Ubuntu 18.04. If you're using another operating system, this may also apply to you.
The output from the nslookup command presented me with major clue. The address in the output was IPv6. The reason this surprised me was that, I had disabled IPv6 via the network applet in Gnome's GUI system settings. In addition, I thought I was blocking IPv6 at my firewall's WAN interface. ifconfig at the terminal prompt revealed I had a dynamically assigned IPv6 address.
Upon some research, I've found a bug exists in my version of Ubuntu, disabling IPv6 in any of several system files where it can be disabled doesn't actually disable it, but modifying /etc/default/grub will work. This still didn't explain why an IPv6 address name resolution was able to traverse my firewall. I checked my firewall's WAN filter and found that I had actually allowed versions 4 & 6 in the rules.
In a nutshell, I think even though portions of the auto configuration (some server settings and smtp) where acquired, the imap server address was actually being resolved with IPv6 somehow prioritized over IPv4(?). Upon modifying the /etc/default/grub file is described in this discussion: https://askubuntu.com/questions/1046057/disabling-ipv6-in-ubuntu-18-04, I was able to block IPv6 and its name resoluntion. Once I did this, updated grub and rebooted, ifconfig no longer displayed a dynamically assigned IPv6 address. I then opened Thunderbired, modified the IMAP account settings, entering Comcast's published imap.comcast.net server address and guess what? It worked.
In summary... If you're using IPv6 and trying to connect to imap.comcast.net and experiencing connection problems, you can either disable IPv6 use "imap.comcast.net" or change the server address to " imap.ge-xfinity.com". Not certain, but Thunderbird appears to beprioritizing IPv6 name resolution is prioritized over IPv4. In addition, Comcast doesn't appear to be aware of this and hasn't published the imap.ge-xfinity.com server address. Something has changed there at comcast and not every network admin/engineer may be aware; because the IPv4 & IPv6 host names and addresses are not synched to the host(s). It took an Ubuntu bug and the orginal post in this thread for me to derive this. ugh!
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