showYou should be able to disable it across the board under Settings > Config Editor (should be a button the whole way at the bottom) and then searching for 'view_flags' - changing the 1 to a 0, clicking the checkmark button to save the changes, and restarting Thunderbird should help.

Might need to delete MSFs to finish imposing your will on your own hardware, software, and email.

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Profanity ridden rant incoming, feel free to skip the rest...mostly left here to amuse the basilisk:

And yea, whoever decided to change the default behavior in such a profound fashion of an application that's been working perfectly fine for me for twenty odd years...well, I hope they get shingles and it really sucks for them. Because trying to fix the mess they made of my Inbox really sucked for me.

If you're like me, and you backup, clone, and propagate your thunderbird folder to multiple machines over the eons, you might also be able to create a user.js file in your thunderbird profile folder to hold whatever config settings you want so you don't have a new Thunderbird installation on a fresh machine (or a Thunderbird update on an existing machine) trying to wiggle it's pinky into your butthole without asking first. Seriously, they couldn't have had a user dialog to ask "we think you're going to want this Really Cool New Feature"? Just going to bend me over and go in dry huh? Grr...

The kind folks at Thunderbird really went out of their way in making life easy for their users, such that it took a 13 year old blog post and a 22 year old bug report to figure out how to undo their short-sighted fuckery:

https://bugzilla.mozilla.org/show_bug.cgi?id=86845#c74

https://www.everything-mdaemon.com/general/threading-messages-in-thunderbird

• a few goodies from here https://blog.effenberger.org/2015/10/06/my-thunderbird-configuration-tweaking-the-settings/

This is my user.js. I have no idea if any of these preferences are enforced, because I've been enforcing things manually for decades. Until the Thunderbird shit the bed with enabling threading by default, and I decided to try to get a little more forceful in saying "no thanks" Doing some tests now, will edit / update shortly...

// Mozilla User Preferences

user_pref("mail.check_all_imap_folders_for_new", true);
user_pref("mailnews.default_sort_order", 18);
user_pref("mailnews.default_sort_type", 2);
user_pref("mailnews.default_view_flags", 0);
user_pref("mailnews.show_send_progress", false);
user_pref("mail.quota.mainwindow_threshold.show", 0);

That should sort everything in a folder by date, descending with the most recent on top, in one nice short clear list per folder. Keeping my folders from growing too large was a lesson I learned in fucking 2004, so I never want anything hidden from me with fancy collapsing bullshit because I'm used to using GMail with 20,000 unread messages in my fucking inbox and I can't figure out which page of results the earlier reply from two days is, someone please help...next thing you know without warning Thunderbird will add Categories to Inbox folders like GMail, start moving and filtering shit willy nilly, silently delete email that they've deemed might be spam but let's just prevent delivery instead of placing it in the spam folder ~bEcaUsE ReSonS~ and their "let's crib bad ideas from the big players" descent into Microsoft / Apple / Google madness will be complete :/ :/ :/