Firefox 29 has been released, and the most prominent new feature is an entirely new user interface. It's smoother and less angular, and has clearly been designed to somewhat resemble Google Chrome. Hence, I personally think it's a major step forward - except for Firefox' version of the Chrome menu, which uses a grid of icons instead of a list (?!) - but I'm nearly 100% convinced many Firefox users will not like it. It's change, after all.
Luckily, Firefox is customisable to the point of insanity, so I'm pretty sure you can revert to the old look with the right themes and extensions.