What is it that you do not understand? The URL you mention explains what is displayed with two-rows, and also with three.

As per my screenshot. Click 2 row / click three row / click kebab. None of these are visible in my Thunderbird

I only see three rows: accounts,trees, list top, email bottom, and Events.

In the middle I see a collapsed card of an email thread, at the bottom the meta of all emails in one thread when selected. But I want to see one specific email body.

OK not very intuitive but this is the ui:

• In cards view only the sender and subject of the original email are visible in the cards list
• clicking on the card only shows the preview
• the preview shows the sender and the first three lines of the emails in the thread, not the actual message
• the card also shows "<nr> replies >" in a very dark grey

Clicking the later, the "3 replies>" will show the messages belonging to the thread. Now each message is available.

It would be nice to include that in the description or think of a inutitive way to do this. Why not a second click on the card to reveal the messages?