Okay, it's doable, but not easy. 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 creates a crude video that walks through the steps at

   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.

Hi Thanks for your time. As I expected there isn't a magic switch.

(Off topic) I actually did some of the above in a previous version of Thunderbird, but last time Thunderbird updated to current version it munched up the login process for Outlook accounts, so that for the default account I could receive but not send emails. No one could tell me why, or how to fix it. So I started again reconstructed everything and got back the threading problem!

Okay. Here's a little-known trick: If you want to be able to reinstall Thunderbird at some future time and not have to diddle with the many config editor stuff you've entered on prior installation, you can create a file that reinstalls on future versions. Here is how to do that: - exit TB - open the prefs.js file in the profile with a text editor, such as Notepad. Do NOT use a word processor, as this is simple text and you DO NOT want to change it. - locate entries you want to use on future installs, such as mailnews.default_view.flags and copy that one line, including the ; at end of line - paste it into a new txt document and save, renaming the file as users.js - Now, when you install THunderbird in the future, copy this file into the profile BEFORE you create accounts and the default will be unthreaded at the outset. :)

Thanks could be very useful.... and would have saved me quite a lot of time