That site (live.mozillamessaging''.''com) is used for the little welcome message that appears below your Inbox when you first start Thunderbird. Certificate errors in Thunderbird can be caused by a range of issues, similar to those affecting Firefox. These include security software that filters your browser connections (e.g., Avast, BitDefender, ESET, Kaspersky), parental control software like Microsoft Family Safety (to test by turning it off, see: [http://windows.microsoft.com/en-us/windows-10/turn-off-microsoft-family-settings]), and/or malware. (Malware was the issue in: [https://support.mozilla.org/questions/993843 live.mozillamessaging.com uses an invalid security certificate. The certificate is not trusted because no issuer chain was provided.]) If you use any of those security programs, Thunderbird probably can be set up to work with their filtering, which also may help avoid missing content in HTML-format messages. Do any of them ring a bell?