I tried Thunderbird recently, and was baffled that there seemed to be no way to see received emails grouped with my responses to them (aka threading or conversations). Even grouping incoming emails that have the same subject seemed like an experimental feature.

Surely I'm missing something? How are people using it? If someone replies to you "I think there was a problem with your attachment", do you search for your sent email?

It’s been there for a while https://support.mozilla.org/en-US/kb/message-threading-thund...

Thunderbird 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...