The Thunderbird Conversations add-on by Jonathan Protzenko enables a sophisticated conversation view in Thunderbird plus a whole raft of other goodies. You can read more about the story behind the add-on here and download it from here. It’s a fantastic option for email power users and for those wishing to directly preview PDF content in Thunderbird:
When Mozilla make the tagline of Thunderbird “Reclaim your inbox”, they’re not kidding. If you don’t want to have a conversation-based interface like Gmail’s rammed down your throat, then you can use the stock standard interface. But if you want a conversation view par excellence, this is an add-on that will fit the bill, and then some.