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.
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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.