I have been using Thunderbird for years to handle my email. In the last week I can no longer send emails. I have reviewed the settings...the server name, the port, password, et al. I even tried re-installing it...no joy.
I have gone thru the troubleshooting Comcast has as well and using Outlook on Comcast Business, I can send both my email accounts tests.
Honestly I am not and never have been a Monzilla Thunderbird user. However, check out this website for some possible Thunderbird technical support that might help you out.
Also, log into your business class account using your admin email then under Manage Services, click Email then hit View DNS settings and this will provide you the list of email settings and addresses you must be using. POP incoming port should be like 110, SMTP outgoing POrt should be either 465 or 587 with SSL authentication.
If your Thunderbird is not working then better to migrate your files into Outlook PST file. According to my experience you should utilize MailProPlus Thunderbird to Outlook Migration Tool. This software can export your Thunderbird Files in seven different file types. You should try it. For more Info: - http://www.mailproplus.com/email-migration/thunderbird/outlook.html