Rated 5 out of 5 stars
Extra Folder Columns is built in now, but in order to enable it, you must modify the key using the config editor. Go to Preferences/Advanced and select with config editor. Search for : mail.folderpane.showColumns
By default, the value is set to "false". Set the value to "true" (toggle on "false"). Restart Thunderbird. You will find the icon for the extra columns is in the right upper corner of the map panel (select unread/total/size).