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:

  In the Message List view, in the column header, on the far right, open the options button that shows    
   context menu. (See attached screenshot):
  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.

This isn't a perfect solution, but it does address the issue of unthreading many folders.

As i mentioned before i already knew this solution and it works only for one folder. And it also works til you stop a Thunderbird. After you relaunch it - all folders become as it was - threading is on again. And your second solution doesn't work either for an old folders. My users have at least 20 folders each

@mediumgap

you say that threading is turned on again after restarting Thunderbird. This is not what I see, did you see the problem yourself ? Did you check that your users have the profile folder off-limits for the antivirus software ?

@mediumgap you say that threading is turned on again after restarting Thunderbird. This is not what I see, did you see the problem yourself ? Did you check that your users have the profile folder off-limits for the antivirus software ?

I've tested it on a few PC's with different antivirus installed. Yes, i saw it myself.

Can you do the following experiment to test persistence of this threaded attribute in your setup:

create in local folders a new subfolder with a characteristic name (unique), maybe test1234. Exit Thunderbird. Goto the profile, copy the file test1234.msf (search for it if you don't know where to search, that's the point of giving it a characteristic name) to a temp dir. Start Thunderbird, switch the threaded indicator for this folder. Exit Thunderbird. Goto the profile, copy the file test1234.msf (search for it if you don't know where to search, that's the point of giving it a characteristic name) to a temp dir, with a different name such as test1234_2.msf Start Thunderbird -> now if the change has taken, that is, the modification you did has persisted, please stop here and report, since it would mean that for some folders the change is persistent and other no, that would be another thing to consider, in this case you should look at the folders where change do not persist and what they have in common.

If as you said, the modification has been ignored, continue with:

Start Thunderbird, switch the threaded indicator for this folder. Exit Thunderbird. Goto the profile, copy the file test1234.msf (search for it if you don't know where to search, that's the point of giving it a characteristic name) to a temp dir, with a different name such as test1234_3.msf Now in the temp dir, do have the 3 files test1234.msf, test1234_2.msf, test1234_3.msf have the same size and (possibly) modification date ?

If yes, please report.

If not, can you open the last file (say in my example test1234_3.msf in the temporary directory) and post the 30 last lines ?