From MozillaZine Knowledge Base

This article was written for Thunderbird but also applies to Mozilla Suite / SeaMonkey (though some menu sequences may differ).

Thunderbird will normally open the next message if you delete the message that you are reading. This is due to it selecting the next message as a sideffect of deleting the current message. If you don't like this behavior there are several workarounds:

• Install the Unselect Message extension. That will change the behavior of the delete button (and delete key) both in the folder listing, and when reading a message in a window.

• Add the "Delete!" button to your toolbar from the Buttons! extension. Use it to delete the message.

• Don't select the message you want to delete. Instead, right click on the message in the folder listing, and select "delete message" from the context menu. This doesn't work if you are configured to preview messages (you enabled the optional message pane in View -> Layout).

See also

• Next and Previous buttons do not work correctly