TB Supernova View/Density setting simultanously applies the same setting to both the Folder-Tree and the Message-List/Thread Pane (at least in Windows classic table view. It should be obvious that much more space is required to see and track a message list which changes every minute and can grow to many dozens of items, while a much more compact view can be used to send and track a folder-tree that seldom changes and is usually much shorter. [And because it 's more compressed, such a folder-tree can usually be seen whole without scrolling.]
This was obvious to the TB community. For decades, previous versions of Thunderbird showed the Folder tree in a compact view whose row height was different from, and more compressed than, the row-height in the Message-List / Threads Pane. It remains obvious today.
This request can can be met in two ways.
Please either 1) Allow the View/Density settings to apply independently to the Folder-tree and the Message List (at least for Windows table view);
OR 2) Compress the row height for the Folder-Tree pane to a fixed "Compact" setting, with View/Density options that change row height only on the Message-List / Threads Pane in Table view.