Do you use default threading for your messages? Or have you reset to unthreaded? I ask because I am not familiar with how search may use threading. This URL may assist: https://support.mozilla.org/en-US/kb/message-threading-thunderbird

It's clear to me that the majority don't want threading to begin with. Can't Thunderbird be distributed with threading off for those who don't want it?

Only in our dreams. I agree that it's not popular, but I've been told that it is non-negotiable. The only solution I have is this:

Do this after installing a fresh copy of TB, but WITHOUT accounts yet. 
- 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. 
- keep the user.js file and insert anytime you do a fresh setup. 

Thanks, I fixed it for myself but I see this as an unnecessary burden on users. I'm all for improvement but I think that the first thing one has to do is ask the users what they want. Then you can go from there. They just took it upon themselves to make the change without asking anybody. There you got it my $0.02.

They also just decided that all new installs of TB will default to the card view of message list, not the table view that is popular. Go figure.

"We" (programmers) know what's good for you. You don't know what's good for you but we do! Jeez, for Pete's sake, let the users decide what they want.

I tried the suggestion by David pointing to the webpage for turning on and off threading, but my appmenu | View does not have a "Sort by" item, where do I find this?

View has only Toolbars, Layout, and Folders, and none have a "Sort by" submenu?

Also tried the mailnews.default_view_flags but that didn't change anything I could see.

Did implementation of Unified Folders change this?

click the 'alt' key and the menu bar should appear. It has all the view items. The config editor change just changes default for new folders, not current ones. No, threading has been around since ver 102.