Some like threading, many don't. I have providered here the info to disable and also the info to prevent in any future installation for you:

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, 
- 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);
- install Thunderbird and exit prior to setting up accounts
- copy the user.js file into the profile
- now, when you start Thunderbird, unthreading will be the default. Problem solved. 

This is still a problem. Every time I turn around, threading is enabled. Why??

• Is the developer so insecure that people don't want this "feature"?

• Why keep throwing it at us. I tried it, I hated it. Let me live in peace!!

You do it you way, I'll do it my way.

PLEASE - LET ME TURN IT OFF AND LET ME KEEP IT OFF!!

Have a look in your profile folder and see how many copies of prefs.js you can find. (There is a button in the troubleshooting information on the help menu to show the folder) There should be one prefs.js. Those that have loads of trouble saving settings have hundreds and need to get whatever is preventing the saving of settings on exit out of the way (Hint: This is usually an over helpful antivirus product that needs an exception for the Thunderbird profile folder to stop it scanning files that are ostensibly in use.)