A bit dated, but a good reference: https://support.mozilla.org/en-US/kb/message-threading-thunderbird You can set this at settings>appearance (which was not available when above article was written)

Thanks - but that's the one resource I had (tried to) linked above. (broken link corrected) It says nothing but (summary): threading is super helpful ! Push here to turn it on , or off, or upside down.

I disagree. This paragraph is on that page and seems to explain the issue well:

Threading puts email conversations together hierarchically. This is especially handy to keep conversations on mailing lists organized, with a lot of people replying to various messages of a conversation. Also, it saves you from losing individual messages of a conversation in stacks of other messages, and lets you perform mass actions like deleting or ignoring a whole conversation.

So: How would I use it ?

People across the web report:

• Visibility of new or unread messages: Users complain they “miss” new messages if the thread looks like nothing changed and they don’t see any indicator. When threads are collapsed, it may be unclear whether a thread has new/unread replies, unless one expands it.

• How to go about Sent messages ? They are not grouped as part of the thread ? A mysterious “open in conversation” or similar feature ? This irritates people who expect a thread to be “everything around a topic” .

• It is not always clear to users what exactly “threaded”, “unthreaded”, “grouped by sort” mean in practice.

• Bulleted list itemSome menu items/settings are buried or behave unexpectedly. E.g. when new messages arrive, threading may re-enable or the view may change in ways users didn’t choose.