I discovered both iptables and nftables were installed. The Debian Wiki
indicated with Debian Buster (10) and onward, nftables will be used.
I attempted to remove iptables and it also wanted to remove connman and
connman-gtk. Removing iptables without dependencies will result in
connman being listed as a broken package.
I was then able to removbe nftables, which did not have dependencies,
keeping iptables installed. However, re-enabling IPv6 in Thunderbird
after all this, did not correct this problem and both it and Claws Mail
remain unable to connect to Comcast's IMAP server.
I have other IMAP servers configured in both Thunderbird and Claws Mail
and neither have a problem connecting to all those other non-Comcast
servers. Comcast's IMAP server has both IPv4 and IPv6 addresses in DNS,
but the other IMAP servers I access only show IPv4 addresses.
