Just to clarify, this is referring to emails received, when displayed in classic view layout.

Are you really referring to the "tree-table-headers" at the top of the thread pane (see picture)? Can you post a screen shot of your "From", "To", "CC" and "Subject" fields which you want to modify? Why only in emails received (Inbox folder) and not also in sent emails (Sent folder)?

So its for all emails. I mean when viewing either inbox or sent, not when composing. Not sure what its called but I marked in red in the attached picture.

I find this layout takes too much screen space, and would like to make it more compact and also change font size for subject.

You don't use the classical layout but the vertical view. What you referred to by the "From", "To", "CC" and "Subject" fields is called the message header pane. You can diminish its height by installing the add-on "Compact Headers" https://services.addons.thunderbird.net/az/thunderbird/addon/compact-headers/

Try to get familiar with the View menus, perhaps you also prefer the "table view" to your actuel "cards view". This SUMO article about "Getting Started with the Thunderbird main window (Supernova and newer) " https://support.mozilla.org/en-US/kb/getting-started-thunderbird-main-window-supernova should help you to get along with the new interface or go back to the UI of earlier versions

Thank you for the above reply and suggestions. The recommended extension is currently not supported by TB 138, but I suppose it will be eventually. However, in case I want more detailed control over the message header pane, is there any way to customize it, maybe by using css?

Basically, this is what I would want it to look like (kindly ignore the missing recipient, this was deleted by me)

You misread the "Compact Headers 5.6" compatibility! Works with Thunderbird 115.0 -* means that this version of the add-on is compatible with TB 115 and higher versions

Basically, this is what I would want it to look like

That is what I already have been talking about. On your first screen shot you use the new standard "Cards view" in your thread pane, in the last one you are using the "Table view". To switch between both views you need to open the small icon on the right of the Quick Filter button. I have put a red circle around it on your first screen shot. I also show you a picture of my customised message headers

First try "Compact Headers" and then tell us what you still want to modify

As for what I am asking, I may have been unclear before: I am talking about the message header pane. Kindy look at the image where I marked in red the area which I wish to customize as described above. I want to avoid this extension as the permissions required seem too broad for what it does. Is there any way to customize myself?

Again, thank you for you time.

No, you haven't be unclear and I've understood that the message header pane is too big for your requirements. That's why I advocated twice to install Compact Headers but you don't like it.

I have spent two hours trying to write a CSS code with which you could diminish the height and the number of lines in the header pane, but I am no developer. I can suppress the "To" box without loosing the date/time box but then I fail to put subject box and date/time box on the same line. I also have a CSS code to diminish the height of the header pane but this would hide the lower part of the pane contents. Sorry, why should I pass my time on this issue when a developer has already written the perfect add-on to do customise the header pane?

As for what I am asking, I may have been unclear before: I am talking about the message header pane. Kindy look at the image where I marked in red the area which I wish to customize as described above. I want to avoid this extension as the permissions required seem too broad for what it does. Is there any way to customize myself? Again, thank you for you time.

Compact Headers installs and works here in TB 138/W11. Its permissions are standard for any other add-on, it is validated before public release, and is no different from the permissions for TB itself.

An example that sets the font size of the Subject in the Header Pane, with userChrome.css:

#expandedsubjectBox{font-size: 12px !important;}

Thanks a lot. Cheers