There is a little work to do, but I also include a means to avoid it in any future installation of Thunderbird. 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. 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. 

Can't seem to access the video link, thanks. Tim

That's ok, as it just demo's the individual steps. I did it as further info, but nothing new in it.

David, can you answer the first part of Tom's question, please: "I've searched for 102.15, but cannot find a way to download it."

I'm wanting to re-install 102.15 because what's now installed is not acting properly.

Thanks, Gail

The latest 102 is here: https://ftp.mozilla.org/pub/thunderbird/releases/102.15.1/

Is it safe simply to download and install this one? do I need to uninstall what's in place before installing the download? thanks yet again, Gail

Try this:

- Click Help>troubleshootinginformation and scroll down to 'profile folder' and click 'open folder' - exit Thunderbird - Delete compatibility.ini file from the Profile folder. - locate the downloaded 102 install file and install over the current version - relaunch Thunderbird

The URL you provide for "the latest 102" is an index of releases. Where can I download 102.15.1, please? Thanks, Gail

Well, it was a list of ONLY ver 102.15.1, showing all the operating systems and languages. I am often guilty of assuming users are English-speaking, in the U.S., and use 64-bit PCs. That URL had the location for Windows version, Linux and Mac and all available languages. BUT, I will assume you speak English, use Windows, and want the 64-bit version. That is here: select the EXE versions: https://ftp.mozilla.org/pub/thunderbird/releases/102.15.1/win64/en-US/

Got it, thanks!

You're very welcome.