Mozilla support forum came through:

Terminal command "nslookup imap.comcast.net" yields imap.comcast.net    canonical name = imap.ge.xfinity.com.

Substituting imap.ge.xfinity.com for imap.comcast.net in the Thunderbird email account "manual setup" window works properly as it should.

Hopefully I'll achieve similar results with Kmail and Sylpheed.