You will have to add the button from the new navigational menu. Clicking on it popup a menu as seen here. Clicking 'Forget' will close all tabs, remove the last 'x' minutes/hrs of browsing history, delete recent cookies, close firefox and open a new fresh window.
This should be useful in an cyber cafe, where once you are done with browsing, you can clear all tracks. Or at home where several people use the same laptop/computer. This feels like a shortcut to 'Incognito Mode', but you do not have to open a new window.