Personally I have Thunderbird place my replies in the folder the original email is in. That is in account settings > copies and folders for each account.

There is also the conversations addon. https://addons.thunderbird.net/en-US/thunderbird/addon/gmail-conversation-view/?src=ss

As for a native conversations view, it is proposed but not yet delivered. See the entry in the roadmap for a global message database https://developer.thunderbird.net/planning/roadmap

I'm 100% positive Thunderbird already HAD a function like that. You right-clicked a message and chose "Open in conversation view". I've been using it for years and now it's missing. When and why was it removed?

Oh, It got moved to a submenu. Right click > Open > Open Message in Conversation

I can see 'conversation view' for individual conversations but there used to be an option to make that the default setting in the INBOX. So all incoming messages were attached to all the previous conversation - which is very useful! I can only change the setting to see the waterfall of incoming messages (with none of my replies) in the conversation (not useful in the slightest way!).

I can see 'conversation view' for individual conversations but there used to be an option to make that the default setting in the INBOX. So all incoming messages were attached to all the previous conversation - which is very useful! I can only change the setting to see the waterfall of incoming messages (with none of my replies) in the conversation (not useful in the slightest way!).

See https://support.mozilla.org/en-US/kb/message-threading-thunderbird#