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.

Threading as implemented in Thunderbird and in the context of this discussion is the grouping of related messages in the same folder.

If you want grouping of messages across folders, you can create a saved search aka virtual folder, for example a virtual that contains both inbox and sent folder messages. https://support.mozilla.org/en-US/kb/using-saved-searches.

When we get new code later in 2026, the problems you describe should be solved.

Thanks Wayne.

My summary, overall, the rollout of threading to me feels like a deja-vue of the "New Outlook". Users forced to adopt, when features are weak, missing or with many known issues. Actually, with Thunderbird I've been looking for an alternative to such annoyances.