One new feature I really like in the upcoming Thunderbird 3 tree : thread summaries in the message view pane. Here is an example:
Click the image above for a full size view.
What makes it even more cool, is that since Thunderbird can already recognise and mark the quoted portion in followup replies, it conveniently ignores this quoted text in the summaries.
I absolutely love the Thread Summaries. 
I wish there was a way to customize thread summaries to show more lines than just two. 
Comment feed for this post