alt Hacker NewsThunderbird supports threading in the message list pane (which displays replies next to the email that was replied to), but a conversation mode for the message pane is still in development:
https://connect.mozilla.org/t5/ideas/true-threaded-conversat...
In the meantime, the Thunderbird Conversations add-on provides a conversation view that looks like classic Gmail, which is probably what you're looking for:
https://addons.thunderbird.net/en-US/thunderbird/addon/gmail...
I had seen both of these. The add-on implementation is too complicated and affects too many systems (contacts, attachments, etc) that change frequently, so it had many subtle bugs and unimplemented features: https://github.com/thunderbird-conversations/thunderbird-con...
As for the Message List Pane, maybe that's something I missed, I'll take another look.