The change to threading began with ver 102 in summer of 2022. I regret that you are now just encountering the switch. Some like it; some don't. I have provided here full info on turning threading off:

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.

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To set many *existing* folders, follow these steps: (I creates a crude video that walks through the steps. The video was created with table view for the message list pane.

   https://davidkirk.org/SettingDefaultFolderView.mp4  that demonstrates the following steps:

   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    
    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. 

Hello David,

1. 1) HAPPY NEW YEAR!!!
2. 2) Your instructions, well actually MOZILLA instructions for unthreading are not accurate. I have 105 in Windows 10. The first instruction says to press the icon up at the top .... the 3 horiz. bars. I did that. A box opens, I clicked on View. In this box, there are 3 options: Toolbars, Layouts & Folders. Clicking on each major header in order, when the new box opens, there is not one item that refers to Threading or Unthreading. By the way, in the Folders box, the 'Classic View is checked.

I tried to follow your directions, but it got too confusing, and I couldn't find some of the items that you indicated. I did not bother to run the video, since I knew beforehand that I would get even more confused. What I don't understand is why MOZILLA feels the need to change a user's settings such as this, when new updates are sent out. This makes no sense to me whatsoever. In the Release notes, it seems to me that an item should be included in the list that now a user has the option of doing 'this or that'. Then a person can make a decision to stay the course or to try the new item. Unfortunately this seems to be the frequent situation, and I've had to deal with the results all too often. So, I don't know where to go from here, sorry to say. At any thank you for your continued attempts to help. robert herold

The first instruction says to press the icon up at the top .... the 3 horiz. bars. I did that. </blockquote> Check

A box opens, I clicked on View.

Why? The instruction is fairly clear I would have thought. The instruction a point 1 state. "Click the menu button Fx89menuButton and choose Settings.

Perhaps read them again, because I really do not have a method to make them clearer. https://support.mozilla.org/en-US/kb/message-threading-thunderbird#w_changing-default-threading-mode