I've just now noticed the Threading automatic default grouping/marking of messages, and it is appalling. Thunderbird/Mozilla either needs to eliminate this (with appropriate warning notices, perhaps) or at least give us the ability to NOT link various messages. It's not only vastly annoying, it can cause severe real-world problems if anyone not in the CC or BCC fields is receiving these messages that were not intended for them. I suppose for now, the only way to NOT link things that were never meant to be would be to change the Subject when discussing important topics. Either that, or get a whole new mail service, which I'd prefer not to have to do.

I started out thinking. This is simple. Link to the appropriate support article which includes advice on how to turn off threading for existing and new folders. https://support.mozilla.org/en-US/kb/message-threading-thunderbird#

But then I got to;

It's not only vastly annoying, it can cause severe real-world problems if anyone not in the CC or BCC fields is receiving these messages that were not intended for them.

You do realize I hope that threading only shows mail already on your computer. It does not send additional links to other emails to other folk, unless you include them. It will thread mail you send and mail someone you communicate with also sends them if the mail is from the same thread. SO if you send an email to an employee saying "good idea" and then forward the original email to HR saying fire her and they use forward to send the person a dismissal notice and remove the visible parts of the email, then it will thread the items together.

I suppose for now, the only way to NOT link things that were never meant to be would be to change the Subject when discussing important topics.

Threading uses the references header. It has been an integral part of email for more than 20 years, it is just more folk are being exposed to what it contains now that threading is on by default. it is not something that can be turned off. The display of threads can be changed and I linked to that article at the beginning.

Fundamentally if you do not want your next message threaded with the conversation, use the Write button and write a new email, then address it and send it. Do not use forward, do not use reply, they retain the references header from the mail you fare forwarding or replying to. You can use edit as new if you want to use a previous email as a base. You can also quote a previous email by having it open in the Thunderbird application and selecting quote message in the menu of the write window but those are not really threading, just ways to get information into a new email.

The answer is helpful because it explains the behavior of Thunderbird or perhaps the way it shows threads. So maybe it is best not to forward, instead write a new email and copy and paste if you only intend some of the message to go to someone else (other mine fields aside like bcc etc.).

That said, another email program "unstacks" or ungroups the threads if I choose to turn off threading. At least that's how I recall or seem to be used to.

In Thunderbird I forwarded and edited a Christmas card boilerplate and thought I had embarrassed myself.