Mozilla releases Firefox 4.0 release candidate | Browsers & Add-Ons | Macworld: The user interface has been completely revamped and streamlined, with the menu bar condensed under a single button. The JavaScript Engine has been overhauled for speedier performance. The Add-ons Manager has been upgraded to a full-page interface.
Tab management has also taken a great leap forward. Users can bundle sets of tabs into different groups, under a feature called Panorama. They can add small permanent tabs to the top of the browser, for those pages and applications they continually keep open.