That shouldn't be happening, but I will share how to at least set the defaults: - click settings>general and click the 'config editor' button - look for mailnews.default.sort_order, mailnews.default.sort_type, and mailnews.default.view_flags - set each according to you preferences, which I anticipate would be -- 2 for sort_order (descending) -- 18 for sort_type (date) -- 0 for view_flags (unthreaded)

. choices are here:

sort_type:
  byNone        17         byPriority    23      byLocation    29 
  byDate        18 *       byStatus      24       byTags        30 
  bySubject     19 *       bySize        25       byJunkStatus  31 
  byAuthor      20 *       byFlagged     26      byAttachments 32 
  byId          21 **      byUnread      27      byAccount     33 
  byThread      22         byRecipient   28      byCustom      34 
    * = commonly desired values
   ** = by Order Received (?)

sort_order:   1 = ascending,  2 =  descending 
mailnews.default_view_flags    0=unthreaded, 1=threaded

The above set the default, not current folders. You may need to reset current folders, but this should prevent being reset to the undersired values.

Thank you for the reply. However it should not be necessary to go to Config Editor level to solve a user interface issue. In fact Thunderbird's own advice is: The Config Editor is a powerful tool, intended to be used by software developers.....Modifications made are unsupported and are used at your own risk. it is possible to make Thunderbird unusable by changing preferences in the config editor". So you can see why I am reluctant to go down this path. Surely there is a way to solve this problem without asking users to do things they are advised not to do? I recently went through every one of my folders and subfolders to specify Descending/Unthreaded view. A few days later they were all back to Ascending/Threaded. This is unacceptable.

I fully agree and have communicated in writing that concept of removing the need for technical interaction, but there is no indication that the decision-makers are open to fixing some issues, so users are forced to fix things themselves. A comment: if you set to unthreaded and days later find the folder reset to threaded, I suggest you check in Windows File explorer the prefs.js file in the profile. If there is more than one, that would indicate that a Thunderbird update may not have completed (usually due to anti-virus software), which causes Thunderbird to revert to defaults.

Thanks for the reply and suggestion. Still not happy with fiddling around with settings at this level. Simpler to switch email clients which I think I'll do.

There is a 100% clone, Betterbird, that defaults to unthreaded.

Thanks for the tip. I have downloaded Betterbird and set up my account. The Sort By settings are exactly tbe same as Thunderbirds, I.e Ascending/Threaded except for those which I have reset to my preference. If I reset all my folders again within BB can I be confident they will stay put?

Yes, that is the default in Betterbird.

I have installed and set up BB and will run it in parallel with TB for a while to compare them. So far all is in sync. Also found a Config Editor button in both TB and BB under Account Settings/Preferences which I have modified to my requirements. May have to wait for a TB update to come to any conclusion. Meanwhile I have descending/unthreaded emails via BB.

Thanks

With BB, there are never automatic updates. You must download and install over current release.

I meant I will wait for a Thunderbird update to see whether it reverts to its own default. As long as BB gives me what I want I'll stick with it. How will I know if BB needs to be updated if it's not automatic?

Why does Thunderbird keep changing my default Sort By preferences from Descending/Unthreaded to Ascending/Threaded. This is most annoying. Thank you

Probably a bug that is in the release process at the moment, it was causing the settings to be lost when the folder was compacted, generally automatically and without any sort of user prompt. You could try changing the compact threshold in settings to a larger number to defer the issue while the fix makes it trough the release process.

Thamk you. Since posting this query I have switched to Betterbird which does not exhibit the problem.

Thamk you. Since posting this query I have switched to Betterbird which does not exhibit the problem.

Of course not. I use Betterbird as well. That does not change the facts of the Thunderbird bug.