Mac/Windows/Linux: It's been a while since we discussed Thunderbird Conversations, but the developers have released a new version that's stable, out of beta, and brings Gmail-style conversation view to Thunderbird.
Thunderbird Conversations 2.0 is a new, stable release that works with Thunderbird 5. You still get the benefits of threaded Gmail-like conversations that can be expanded and collapsed in the message window, but the extension now also allows you to collapse quotes, open conversations in a tab, and see contact tool-tips that let you quick-reply, forward, or add a contact just by hovering over their name.
Thunderbird Conversations 2.0 is free and available now. It works everywhere Thunderbird does.
Thunderbird Conversations [Mozilla Labs]