Using Thunderbird email - suddenly cannot sent email, though incoming email is just fine.

HELP!  I have urgent email that needs to go out!  I only work part time and I've wasted most of my already too short of a work day trying to fix this problem.

I'm trying to troubleshoot an outgoing email issue I'm having.  We use Thunderbird, and we're receving emails just fine.

When I try to send emails, I now suddenly cannot send them.  

First I get a dialoge box which says: LOGIN FAILED  Login to server smtp.comcast.net failed.   [ three options: try again, enter password or cancel]

Then I get this, once multiple tries with the prior option do not work (and our password has not changed:

Sending of message failed.
The message could not be sent using SMTP server smtp.comcast.net for an unknown reason. Please verify that your SMTP server settings are correct and try again, or contact your network administrator.

I'm tryign to verify all the settings on your site to make sure they are correct for Thunderbird.  I know that things can change.  However, I cannot find the settings - only ones for Microsoft.  I assume those would be the same, but your website is very very confusing on this matter, and other setting reviews come up with error messages.