Reply and Forward arrows could show the referenced message when clicked
It's a Thunderbird suggestion, not a question.
When clicking on Forward or Reply Arrow, it could shows the respective message below the Replyed/Forwarded message or oppening the respective folder with the message reffered selected.
Alle antwoorden (2)
Topic submitted to the correct language, please wait for a response from a volunteer from your region ...
Not sure if this is exactly what you're looking for, but if you right-click a message, Open Message in Conversation, you can see the received and sent messages in the thread.
See also ThreadVis for another option.