From MozillaZine Knowledge Base

You can add a column to the folder pane to display the number of unread messages, the total number of messages, and/or the size of the mail folder. Displaying the number if unread messages is a useful way to avoid having to select a folder to see if it has unread mail.

Thunderbird 2 and earlier versions:

• Check the box for "Show expanded columns in the folder pane" in "Tools -> Options -> Advanced -> General Settings", and then press the "OK" button. This will add a small square control at the right end of the column headers in the folder pane.

• Click on that control and select Unread, Total, and/or Size.

Shredder (upcoming Thunderbird 3 release):

• Install the Extra Folder Columns add-on. This will add a small square control at the right end of the column headers in the folder pane. You may need to disable version checking or edit the version it supports to install it.

• Click on that control and select Unread, Total, and/or Size.