Thunderbird Menu Button on the Top Right

By default, Thunderbird 17 replaces the Menu Bar (the toolbar with the File, Edit and View menus) with a new Thunderbird Menu button.

A simple click on this button shows all the menus:

To get the Menu Bar back, see Display the Thunderbird menus and toolbar

Title Bar Replaced by Tabs

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.

Minor changes

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:

1. rightcontrol-clicking on any folder,
2. select Properties... and then General information tab.