You mean that it reverts the setting for the same folder, right? I have this issue in my inbox (but no solution). It does not happen often. Since I have other issues with the interface, I wonder if they are related (https://support.mozilla.org/en-US/que.../1427177 ). I do believe that there is a difference between the default setting for new folders and the setting for existing folders, in case that is relevant to you. I wish there was a global setting to never use threaded view.

Yeah exactly what it happens to me. Threaded views shoud not be the default. And of course, once you have chosen a setting, Thunderbird shoud not be allowed to change your choice.

You are right; setting for unthreaded applies only to the single folder. To set unthreading everywhere,

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 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.