Expanded columns in folders pane - Thunderbird
From MozillaZine Knowledge Base
(Redirected from Expanded columns in folders pane (Thunderbird))
You can add columns 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 of unread messages is a useful way to avoid having to select a folder to see if it has unread mail.
Thunderbird 3 and newer:
Thunderbird 2 and earlier versions: