What you're experiencing is "threading"; the powers that be at Thunderbird decided to make threading of emails the default. Many don't like it, so you're not alone. You can fix it for a single folder by highlighting the folder and selecting View>sort by>unthreaded. However, that can be a pain if you have lots of folders. To do multiple folders, here is that info:

More info: In config editor, you can change default that applies to NEW folders you set up: mailnews.default_view_flags 0=unthreaded

To set many folders, follow these steps:

   In the Message List 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.

This isn't a perfect solution, but it does address the issue of unthreading many folders.

You refer to 'config editor' but haven't suggested where I might find that. When I ask Google that question I get reply ' open the 3-bar menu on the right'. Then I think it said look for 'preferences'. There is no 'preferences' option in the 3-bar drop-down menu. One step forward and two backwards, as usual. Come back Outlook Express.

Further experience of this 'threading' - which I didn't ask for - suggests that as a result the emails cannot now be listed in full chronological order. How idiotic is that? Do you think the Mozilla people ever use the new product after they've taken away from us the one we've worked hard to understand? That's the sort of thing Microsoft could be expected to do.

Try tools>settings>general ('preferences' was changed to 'settings' in recent release). Yes, emails can be listed in chronological order.