By default, Thunderbird 17 removes the caption from the title bar, instead displaying the tabs there:
How can I restore the caption to the title bar?
You need to use the Config Editor to change
To do that, click the menu button (marked with a red box in the above picture) and select Options... Click Advanced, then the General tab, then the Config Editor... button. Type
Note: You can restore the classic menu bar by right-clicking to the right of the last tab and selecting Menu Bar.