Thunderbird 3: show total number of items in folder?

"In TB38, this is again standard functionality but hidden in :
View => Layout => Folder Pane Columns."

"See the Number of Messages in the Mozilla Thunderbird Folder List

To display the number of emails (both unread and in toto) for each folder in the Mozilla Thunderbird folder list:

Select Tool | Options... (or
Thunderbird | Preferences... if you
use a Mac) from the menu in Mozilla
Thunderbird.
Go to the Advanced category.
Open the General tab.
Make sure Show expanded columns in
the folder pane is checked.
Close the configuration dialog.
Click the table icon in the folder
pane (next to the Name header).
Click Unread, Total and Size so that
a checkbox appears next to all the
columns you want to see.

Source

In a short way ;)

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."

"You can install this extension for thunderbird.

addons.mozilladotorg/fr/thunderbird/addon/9716"