About this Add-on
Often, you are going to delete a message. The "standard" behavior is to go to next message after deletion, but in the given context above one would like to go to the previous message, to read the "next" unread message.
This is what this Thunderbird addon does, you just have to download it and activate it: there's no setting.
Note that if you just want Thunderbird to select nothing after deleting a message, there's the addon "deselect on delete tb78" (which I used to create this extension).