Is that conversations view? I have a button in the main toolbar to turn it on and off. There may be other places.

In my version, 128.9.2esr (64bit), just go to the the VIEW tab, then Sort By, and then down almost to the bottom, check UNTHREADED and it will fix that. Then every message, whether it's by itself or in a list of replies, will appear as it's own message in the list, and according to how you want it listed, (date, correspondents, subject, whatever).

Hopefully that helps! -S

In my version, 128.9.2esr (64bit), just go to the the VIEW tab, then Sort By, and then down almost to the bottom, check UNTHREADED and it will fix that. Then every message, whether it's by itself or in a list of replies, will appear as it's own message in the list, and according to how you want it listed, (date, correspondents, subject, whatever).

Samuel seems happy with the threading in the message list, but wants to change how messages are displayed below the list. My guess about conversations was wrong. I cannot find a way to duplicate this layout. Samuel, are you using an add-on to modify the layout?

@Scooter: I do not have a Conversations button on the main toolbar, but I was able to add one by customizing it. Well, clicking the Conversations button simply opens the current email's thread (i.e. "conversation") in a new tab. It doesn't affect what the preview pane looks like, though (not in the main window and not in the tab either). In the new tab, clicking on a threads in the list causes multiple emails to be shown in the preview pane.

@Shawn: Disabling threading disables threading, but it doesn't affect the preview pane. Even with threading disabled (i.e. the message list shows only single emails and no threads), clicking on some emails in the list causes multiple emails to be displayed in the preview pane.

@Samuel: According to your screen shot you have installed the add-on "Thunderbird Conversations" https://addons.thunderbird.net/en-US/thunderbird/addon/gmail-conversation-view/ (which has nothing to do with TB's native "Conversation" button). You also have chosen in menu View > Sort by > Threaded as Swan already mentioned. These are your choices. If you don't like the actual view in your preview (message) pane, disable the add-on and/or change your view in the thread pane in "Unthreaded"

@Scooter: Thanks for reminding me to check if I have an add-on installed that might cause this. Found it!

For some reason, I have an add-on called Thunderbird Conversations installed. I don't remember installing it. I installed only the add-ons that my old version of TB had, but, well, I suppose I must have installed this one on some kind of recommendation.

Or perhaps I was trying to find an add-on that reduces the screen real estate of the preview pane's heading -- which is about twice as large in TB 128 than in TB 69. I see now that this add-on tries to mimic some of Gmail's UI features. The add-on does reduce size of the preview pane's heading.

https://github.com/thunderbird-conversations/thunderbird-conversations/wiki

I am glad that you solved the problem or at least found its source.

By the way, the pane that you are referring to is the message pane, not a preview pane. I point that out not to be pedantic but because you will want to know how Thunderbird refers to it in the menu and how it is referred to in this forum.

By the way, the pane that you are referring to is the message pane, not a preview pane.

Thanks, I'm probably giving away my age, then.

When I started using email, that pane didn't used to be there and you had to double-click a message in the list to open it in a separate message window. Then that pane was added to several email programs, to show a quick preview of the message before you open it in a message window.

ChatGPT says TB used to call it a "preview" pane until the mid-2010s. (I wonder how accurate that is.)