Threading has been a real pain since its introduction in 102. No relief is in sight.

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.

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. 

All of David's guidance is very detailed and useful in general threading management, but it doesn't address ryanrsnyder's original issue with Thunderbird failing to remember the View > Sort by > Unthreaded setting for the Inbox. Like Ryan, no matter how many times I reset my Inbox to Unthreaded (with v.115.5.2 in Arch Linux 6.6.7) it magically reverts to Threaded. Very frustrating. Please pass along to the development team that the Inbox setting needs to be pinned down.

Could this be a Linux version issue? I don't see it in Windows. In hindsight, I wish I had saved such details for a comparison. My inference is that the developers see threading as a gift from the gods, a feature that all will love. But it hasn't turned out that way. I will try to gather more data on this. Thank you.

Indeed could be a Linux-specific issue. I'm only one data point, but would be glad to share any of my configuration parameters with you and the dev group if that would help.

And you're right, it hasn't turned out that way. Thanks for monitoring.

caj-bugz & david,

My initial problem does seem to be solved now that I have set mailnews.default_view_flags to 0 via the config editor. It used to reset to threaded many times each day, but it's now been a couple of weeks without that reset happening. The "Sort by" now sticks. Also, my problem is on a Windows box, not linux.

caj-bugz & david, My initial problem does seem to be solved now that I have set mailnews.default_view_flags to 0 via the config editor.

Thanks for posting that information.

I would suspect the automatic compact action is resetting the threading. There is a bug report for this.

FWIW, I'm using TB 115.6.0 on a Mac, and I have the same problem. I manually set mailboxes to be unthreaded, and TB continually sets them back to threaded. I have to reset mailboxes back to unthreaded several times per day, and I share the frustration that other posters have expressed.

I'll keep watching this support thread, hoping to see a solution posted soon.

Posting again - - - hoping that this issue is getting attention from TB developers.

I can't count how many times per day I have to change "Sort by" from threaded to unthreaded across four different email accounts. Very frustrating.

Developers, are you working on a fix for this?