Open In Conversation
Thunderbird add-on to make the default message list action "Open Message in Conversation"
How it Works
First, be sure you have message threading enabled (View > Sort by > Threaded). Then, right-click on a thread. Notice the option "Open Message in Conversation":
Basically, this add-on makes that the default action when you press "Enter" or double-click a message.
This works best when you don't have the "multi-pane" layout (i.e. you open messages in a new tab instead of viewing them in a pane).
The window that opens contains not only the messages in the thread that are in your inbox, but also ones in other folders (so you can see sent messages in there too).
Download thunderbird-open-in-conversation.xpi. Then, in Thunderbird, go to Tools > Add-ons, choose Extensions in the pane on the left, click the gear drop-down, select Install Add-on From File, and choose "thunderbird-open-in-conversation.xpi".
This project is licensed under the Mozilla Public License version 2. For more, see the LICENSE file.