How do I delete or block an abusive contact

You can automatically delete messages from a sender based upon their email address by using Thunderbird's Message Filters feature. This article describes what they are and how to use them: https://support.mozilla.org/en-US/kb/organize-your-messages-using-filters Your email provider probably has similar filtering and blocking features, accessible on their webmail page. The advantage of doing it there is that the problem will be taken care of even before Thunderbird can download it.