How do I restore the caption to Thunderbird 17's title bar?

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 mail.tabs.drawInTitlebar from true to false.

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 title into the Search box, then double-click the mail.tabs.drawInTitlebar line to change the value.

Note: You can restore the classic menu bar by right-clicking to the right of the last tab and selecting Menu Bar.