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", and check the box for "Show expanded columns in the folder pane." Click "OK". This will cause a small drop-down marker to appear in the top-right corner of the folders pane.

• By clicking on this drop-down marker, you can then select Unread, Total, and/or Size to be displayed.