https://addons.mozilla.org/en-US/thunderbird/addon/gmail-conversation-view/?src=ss

that add-on perhaps

No add-on needed. Already in TB

Try Tools|Account Settings (Select an Account) - Copies & Folders (in top section - "When sending messages, automatically:") (tick checkbox) - "Place a copy in:" (enter some location), also (tick checkbox) - "Place replies in the folder of the message being replied to" OK out. valid for TB38.2

gild said

No add-on needed. Already in TB Try Tools|Account Settings (Select an Account) - Copies & Folders (in top section - "When sending messages, automatically:") (tick checkbox) - "Place a copy in:" (enter some location), also (tick checkbox) - "Place replies in the folder of the message being replied to" OK out. valid for TB38.2

I did not suggest that one (which I use myself) as it does not place original mail with replies, only replies to incoming mail

gild said

No add-on needed. Already in TB Try Tools|Account Settings (Select an Account) - Copies & Folders (in top section - "When sending messages, automatically:") (tick checkbox) - "Place a copy in:" (enter some location), also (tick checkbox) - "Place replies in the folder of the message being replied to" OK out. valid for TB38.2

If you want all threads, including ones you initiate to be together, do the above and set your sent messages to be filed in your Inbox:

Tools > Account Settings > [account name] > Copies & Folders Under When sending messages, automatically: select tickbox Place a copy in: Choose Other: and navigate to “Inbox”.

Done. When you start a conversation, the copy of your send message will go in your Inbox, the same place where the replies will come to – so they’ll all be in one thread.