As of Thunderbird v17, Mozilla has taken a page from the Google Chrome playbook by hiding the program's title bar. This is an annoyance because by decades-long habit, many Windows users including yours truly like to double-click a program's title bar to toggle between windowed/fullscreen, instead of zeroing in on the wee maximize/restore buttons in the top right corner.

Unlike Chrome, however, Thunderbird at least has user-accessible advanced settings.
Menu > Options > Advanced > General > Config Editor
search for mail.tabs.drawInTitlebar, double-click to toggle from "true" to "false".