You are not alone, as many of us are displeased with that decision. I see complaints almost daily on the issue.

This is the official (to my knowledge) documentation. Well worth the read https://support.mozilla.org/en-US/kb/message-threading-thunderbird

Here is my own documentation on Threading, duplicating the above, but it may help: You need to highlight each folder and click View > Sort by > Unthreaded. If this applies to just a few folders, just repeat the process as necessary. If you have many folders or plan to add many new folders, you may wish a different approach below:

More info: To change default for *new* folders, In config editor, you can change default that applies to NEW folders you set up: mailnews.default_view_flags 0=unthreaded

Config editor is available at Tools > Settings > General and scroll to bottom right.

 ------------

To set many *existing* folders, follow these steps: (I created a crude video that walks through the steps below at https://davidkirk.org/SettingDefaultFolderView.mp4)

   First, highlight a folder that you have set to Unthreaded	
   In the Message List view, in the column header, on the far right, 
   open the options button that shows the context menu.

   At the very bottom is the option:
   -> "Apply current view to..."
   -> "Folder and its children..."
   -> Hover over an account
   -> In the next menu don't select a folder. Instead select the account name itself.

ALSO, for future installations of Thunderbird:
- install Thunderbird and exit prior to setting up accounts
- create a simple text file with name of user.js with these lines: 
     // Set default threading to UNthreaded  ###   <== this line is optional, just for documentation
     user_pref("mailnews.default_view_flags", 0);
- copy the user.js file into the profile
- now, when you start Thunderbird, unthreading will be the default. Problem solved.

Thanks for the info!

Re the shortcuts, I have found a way to mitigate almost all the pain. There is a keyboard shortcuts extension called tbkeys-lite. That lets you reconfigure all the single character shortcuts to do nothing. When you add it in, and go to the options for it, there is even a handy button "Unset singles" that fills in the config with the necessary stuff to disable all single character shortcuts.

The following is an excerpt from the settings. Note that m is configured to get email messages, which is what I was used to from long ago.

   "m": "cmd:cmd_getNewMessages",
   "a": "unset",
   "b": "unset",
   "c": "unset",
   "f": "unset",
   "j": "unset",
   "k": "unset",

Thank you for the tip. Much appreciated.

This is all caps for a reason... I AM FRUSTRATED!!! I HATE THREADS. WHY DOES IT KEEP GETTING TURNED BACK ON?

I know some people like it, fine, let them use it. But why can't I completely just get rid of it? I think it is some coder's idea of how I should organize myself. Forget it. I organize my self by the calendar not by some weird thread. Sorry. If I could find the person in charge of the wonderful threads, I'd really do some damage. I HATE IT. THAT DOES NOT MAKE ME A MONSTER OR A LUDDITE, IT MAKES ME DIFFERENT.

I Have been using Thunderbird for about 20+ years. I am now, for the first time looking at finding another mail client.

To me, THREADS ARE NOT EFFICIENT, THEY MAKE ME LESS PRODUCTIVE. THANKS.

I HATE THREADS. I HATE THREADS. I HATE THREADS. I HATE THREADS. I HATE THREADS. I HATE THREADS. I HATE THREADS. I HATE THREADS. I HATE THREADS. I HATE THREADS. I HATE THREADS. I HATE THREADS. I HATE THREADS. I HATE THREADS. I HATE THREADS. I HATE THREADS. I HATE THREADS. I HATE THREADS. I HATE THREADS.

PLEASE GROW UP AND LET ME TURN IT OFF!!

I understand your frustration. Threading has been a sore point since it became the default in summer of 2022. The fix for that is listed earlier in this post. Once you have changed the default, it does not reset. However, if you check your profile and see multiple instances of prefs.js, that would indicate problems in updating the settings. You can switch to Betterbird (a clone that defaults to unthreaded), but your profile would need to be unthreaded when switching.

The more of these support entries I read, the more angry and frustrated I become. The thread problem I have is not from a choice I made on the menu. That choice is not chosen. The threads are right on the subjects of the e-mails. I never got a choice and haven't seen anyone talk about a choice that is relevant to what happened to me.

I am trying to assist, but I don't grasp what you are experiencing, since it is not what I think of as threading. You're saying the folder is unthreaded, yet the messages appear threaded by subject. Maybe some other voluhteer on the forum will offer some insight.

Ken,

There is no way that I have found to undo the stupidity from hubris at Thunderbird.

You can keep turning it off until The Wonderful People at Thunderbird decide to turn it on :-)

View -> Sort By -> Unthreaded View -> Threads -> Ignore Threads

Makes you want to find the genius who is responsible and do some physical harm.

p.s.You may need to do this routinely.

Its easy to understand if you HATE THREADS...

Unfortunately, the idiot that decided that we all need to use threads has someting in Thuderbird that keeps turning them back on. UGH!!

View -> Sort By -> Unthreaded View -> Threads -> Ignore Threads

However, if you use

Find -> Search Messages

Its in the glorious state of threaded.

Ever want to yell at the idiot who decided we all needed to organize our lives they way they like to organize their lives? I do. But alas, we have no chance, because they know what is "best" for us.

I HATE THREADS!!