Expanded columns in folders pane - Thunderbird
From MozillaZine Knowledge Base
In the folders pane in Thunderbird, you can display the number of unread messages, the total number of messages, or the size of the mail folder. To do so, first go to Tools > Options > Advanced > General Settings, check the box for "Show expanded columns in the folder pane" and click OK. Doing this will cause a small drop-down marker to appear in the top-right corner of the folders pane. If you click on this drop-down marker, you can then select Unread, Total, and/or Size to be displayed.