Emails come from the sender and they may not use xfinity & Comcast servers. xfinity & Comcast are companies that have servers and offer mail accounts. They would not be tampering with the contents of emails they receive on behalf of yourself. If xfinity themselves did send you an email then the logo maybe contained therein, but in Thunderbird, that logo may not be displayed if it contains remote content. Thunderbird by default does not automatically display any remote content for security reasons. See info on 'Remote content' https://support.mozilla.org/en-US/kb/remote-content-in-messages