I don't know of any formal technique to do this. Here is a trick that worked for me. Place your cursor on the first address. Move the cursor to the very front or beginning of the line. Type in your new address, and then a comma, and then press return. Thunderbird doesn't like multiple addresses on one line, so given that arrangement it will split the line into two discrete addresses. The original first address, which you made second, now becomes last. More directly, you could just cut-and-paste (double-click to select, ctrl+x, ctrl+v) the first address and move it to the first empty box, then re-fill the now empty first address box with the additional address. An advantage with doing it this way is that the auto-complete will still work. No need to retype anything. I am glad I work with folk who aren't sensitive to address order. Note that To: addresses will always appear before any Cc: addresses. So if everyone else is Cc: and you add one at the end as To:, it will be presented first anyway. And also bear in mind that servers will process your message, and you may have no control over what order they present addresses. What you put into your message will be re-written into the what is known as the message envelope