We’ve made thoughtful tweaks to the interface to give Firefox a streamlined feel. You’ll also notice bigger, bolder design elements as well as more space for viewing the Web.
Translation: We tried to hide more buttons and functionality from users with Firefox 40, but in the end people complained about the lack of a field to enter addresses into and the removal of the back button. Users, tsk, tsk... Rest assured that in the future we will continue to add more useful buttons and features like Pocket and voice chat.
The Firefox "UX" Team
In all seriousness the "new look" for Windows 10 doesn't look all that different from FF 38 that I've been using for months in the Tech Preview. I can't wait to try and find what other menus, options and functionality they have "designed" out of FF 40.