Make sure the recipients email address is correct.

All with yahoo are returned. All others go through

What is the exact error message you get?

This is an automatically generated Delivery Status Notification.

Delivery to the following recipients was aborted after 2 minute(s):

Note, I sent them a test email direct from comcast no problem

This is a mozilla issue

The issue is most likely not a Mozilla issue.

The Delivery Failure Notification is about a message from comcasts email severs to Yahoos that is being rejected. Thunderbird's transmission to comcast was accepted or you would have received an error message from Thunderbird when you clicked send. Comcast do not use the same SMTP server to send mail from their web interface. So being able to send from there has no diagnostic value at all really

Now can you please post the whole error message. There is a lot more information in a Delivery Failure Notice and you have left out almost all of it.