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Thanks!!!! That worked!!!!

You're welcome. Some people love threads, some hate them. :) (I hate them...)

The problem is.. the fix only works until the next update and then it reverts. I've recently changed to a new laptop, the old one ( still in operation ) doesn't have the threading problem and I've never had to set it. The new install has the problem and it drives me mad, same op system same version of Tbird so why?

I haven't seen that, as the setting is individually stored in each folder's msf file, not one central place. Thunderbird prior to ver 102 (which was released in summer of 2022), UNthreading was the default. For future setups, to start with unthreading as the default, do this:

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); 

Next,

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