You have said both "auto download" and "auto open". Most of us would consider it normal and correct for an email client to automatically collect ("download") new messages on being started. We would be rather annoyed, and concerned, if it went further and opened those messages and displayed them. There is no setting in Thunderbird to make this happen, so if this is what you mean, something is seriously wrong. To open a message, you have to select it. If you don't want Thunderbird to collect messages for you, look at Account Settings and clear the checkboxes for automatic download at start-up and at timed intervals. However, many IMAP servers now work on a "push" principle and don't pay much attention to these settings. They simply offer messages to the client as and when they arrive. If you want to micromanage the downloading, you'd be most successful if you set your accounts to use POP.