Comcast email servers will not accept IPV6 connections as they have not been upgraded to IPV6. You have to change the settings in Thunderbird, actually down grade them. The standard IPV6 setting has to be turned off, which then reverts to IPV4. Below is a copy of the post from the Ubuntu Help forums:
Comcast servers will not except IPv6.... Disable it in Thunderbird:
Here's how: in Thunderbird, goto Edit>Preferences>General Tab.
On the General screen, select "advanced" tab > "General" subtab, and in the lower right corner find the button "Config Editor" Click on it,
acknowledge the warning, and scroll a long ways down to: network.dns.disableIPv6 (default: false)
Double-click on the line, and the value will change to "true"
Back out and Okay, shut down and restart Thunderbird, and your client will connect to the imap server in a flash.
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