I need to remove Earthlink.net from Thunderbird so that Thunderbird will recognize Comcast.net as my e-mail carrier.

Earthlink and Comcast accounts aren't related to each other. You will need to set up your new Comcast account in Thunderbird. https://support.mozilla.org/kb/automatic-account-configuration

You don't have to delete the existing Earthlink account in order to be able to set up the new Comcast account.

Is there any existing mail in the existing Earthlink account you want to keep?

What is the account type of the existing Earthlink account - POP or IMAP?

jtmcloud said

Earthlink has discontinued their service through Comcast so I had to go to Comcast for internet and e-mail service. Now I can't send e-mails out because the service has been discontinued. I need to remove Earthlink.net from Thunderbird so that Thunderbird will recognize Comcast.net as my e-mail carrier. Sincerely, Jim McLoud