Ubuntu Vibes posted a guide about installing Thunderbird Conversations 2.0 is an add-on for Thunderbird 3.3 in Ubuntu

Thunderbird Conversations add-on is developed by Jonathan Protzenko from Mozilla Labs Team. A look at its features:

A regular conversation view that fetches messages from all folders, and behaves just like Gmail
Integration with Thunderbird Contacts: participants have tooltips in the conversation, and you can view their avatars and profiles from Facebook, Twitter etc
A quick reply feature, with autocomplete, that allows you to reply to a thread in a snap
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  Thunderbird Conversations Addon Enables Gmail Style Threaded Conversation View