I share your pain, as I wasn't pleased with that change either. This may help: At config editor (tools>general and scroll down to bottom, change mailnews.default_view_flags to 0 to make unthreaded the default. I think that only applies to new folders though.

Two things here. Please respond on both of them.

1. Bug 1764842 Is supposed to only apply for new profiles. So anyone upgrading should not be affected. Were you prompted to start a new profile (asked to enter your account details over again) on upgrade?
2. I will offer a little more information that you asked for because I personally have issues with a number of the message list defaults.

Using the config editor you can modify the following hidden preferences.

mailnews.default_sort_order Set the integer value to 1 for ascending and 2 for descending order mailnews.default_sort_type This is a bit of a monster for choice as the sort type can be set to any of the following values. 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 

I would expect you would be looking for setting date so 18.

mailnews.default_view_flags

This would normally be set to 0, but supports 0 (default): None set 1 Threaded display 8 Show ignored 16 Unread only 32 Expand all 64 Group by sort

@David. I have filed https://bugzilla.mozilla.org/show_bug.cgi?id=1787599 in the hopes of resolving some of the issues that appear to be circulating around Bug 1764842. Hence it took me about 2 hours to draft and submit my reply here. I would appreciate anything you can add there.

Bug 1764842 is supposed to affect only new folders yet this and other reports you mention appear to indicate existing folders are affected post update. What is not clear is if there is a regression in the bug, or if other issues are at play causing new folders post update (profile per install perhaps)

I had to reinstall Thunderbird so it's like starting fresh again and it's since this reinstall that this issue is happening.

I picked this tip up earlier today, and it may prove helpful:

Configure a folder.

   In the header view, in the column header on the far right, open the options 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.

I had to reinstall Thunderbird so it's like starting fresh again and it's since this reinstall that this issue is happening.

Why? I have been using Thunderbird for more than a decade (version 2) and I have had issues where I needed to mess about post update once. I don't count when daily builds break stuff, it is the bleeding edge after all.

But why folk reinstall is something I am trying to understand. simply because it should never be required and understanding what people are doing might provide insights into things about Thunderbird that need to be fixed or improved so this reinstall practice ceases.

I had to reinstall Windows, hence the reason I had to reinstall Thunderbird. And whenever I get a new computer I need to install Thunderbird and set everything up again. If there's a way where all of my settings are saved so when I install Thunderbird on a new computer that would awesome... please let me know.

Ver 102 makes it easy. When starting Thunderbird, ignore the email wizard and click the Home tab. On that screen is an 'import' button that will import your saved profile for immediate access, with all settings as they were. It's that simple, and a HUGE improvement over prior versions. There is also a Tools>export feature on main page that will create a full zipped profile backup whenever you move to a new computer.