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.