hello hello

I think you want to Specify whether messages in the message list are grouped into "threads".

You can do this by changing the sort order on Thunderbird

Menu location: View | Sort by | Threaded

This should fix your issue.

-Michael

Dear Michael

Thank you very much for your help. However, what I want is to have all the messages with the same title IN ONE HEADING/TITLE in the upper window, which means you never see two identical titles in the window. But after choosing the option you suggested I still see these messages in separate headings although they are exactly the same. I am still seeing three exactly same titles in a row. I want to collapse them into one. I really hope you understand what I am talking about.

A good example is when you check your email on your phone. When you open the email app, you first see a list of headings. Each heading leads you to all the conversations/messages that went on under that heading. Importantly, in the list of headings you never see the same heading more than once. That's what I mean by "having all messages under one heading". Now I want exactly the same in my Thunderbird on desktop. Does TB support this? If so, how can I make it happen?

You seem to be asking for Thunderbird to default to showing the Threaded view, with the Threads closed.

Use the config editor 'Tools - Options - Advanced'

Change mailnews.default_view_flags from 0 (unthreaded) to 1 (threaded). If you want to expand all messages by default then add the decimal value 32 to the previous value.
from: http://www.badpenguin.org/thunderbirds-default-sort

Close and re-open Thunderbird. Thunderbird now will default to open with threads closed. You can still open and close threads with the menu or by clicking the little triangle in the thread pane.

TB-38.2 Win10-PC