It's official: Firefox 4.0 will sport a brand new UI. But you have to wait until the last quarter of 2010 to experience it. The mockups for the new interface have been posted, and they have not much difference from Firefox 3.7 mockups. It is evident that Firefox is going the Chrome way.
Firefox is a browser. All it needs is speed and an uncluttered interface, not animations or high-def graphics. Also, this new UI may be different for various platforms. This might irk some people. The mockups heavily flout the guidelines provided by Microsoft (see this), just as Google chrome does.