Starting in Thunderbird 17.0, the tabs are drawn in the title bar. The old behavior can be restored by setting the preference mail.tabs.drawInTitlebar to false.

Values

true (default)
The tabs are drawn in the title bar of the mail program.
false
The tabs are drawn in a separate tab bar, and the title bar shows the title of the current tab (e.g. the subject of the currently selected mail).