I understand your frustration; I don't like the default either. But, the fix works. and the solution I posted gives full details on how to ensure a default nothreading for any future setups. Yes, the video I did was a bit crude, and I used a Gmail account only as an example. There were detail steps also included in the post on the actions done in the video. Here is a link to one of the posts of solution:

 https://support.mozilla.org/en-US/questions/1439822   

Your dislike has been shared widely within the Thunderbird community since it became the default in summer of 2022 in ver 102, as this dislike is posted almost daily and sometimes several times a day. Will it be changed? I doubt it. My solution is to create the user.js file as documented in my post and insert that immediately following any future thunderbird installation.

We are just foot-soldiers here, volunteers with no authority for anything. If you wish to share your concerns at a higher level, I suggest/encourage going to connect.mozilla.org and, under Ideas, post your concerns for threading. Thank you.

Thanks for your reply. I have posted a comment on connect.mozilla.org as you suggested. I have also looked at the link you gave and I can get to the sort groups part and unthread folders, but then the next part to unthread multiple folders doesn't work because I can't find the "options button that shows

   context menu. :"

This is an example of unnecessary complications, there should just be an option in an easily found part of Thunderbird e.g. "Settings" to do a global unthread. I can't understand that if people want to make something a default they can't make an easy option to undefault it! They are just annoying and wasting everyone's time.

So you are saying you can't find this little devil.

Perhaps you are using the cards view of emails and you need to change to the table view using the button beside the quick Filter one.

Or if you have that turned off as well just use the entry on the main menu bar. Press Alt or F10 to make what is usually hidden visible and select the view menu followed by sort by. Note this menu can be turned on permanently using a right click on the toolbar of the menu bar and selecting it from the menu.