By default, Thunderbird 17 replaces the Title Bar (the bar at the top that identifies the window) with space for tabs. This is controlled by a new mail.tabs.drawInTitlebar preference that you can change in the Config Editor.
Folder size added to folder's properties
It is now possible to check the size of a mail folder on the disk without installing an add-on. You will find it by following these steps:
rightcontrol-clicking on any folder,
select Properties... and then General information tab.