Mozilla has officially released Firefox 4, a major update of the popular open source Web browser. The new version introduces a much-improved user interface, significant performance enhancements, strong support for the latest Web standards, and noteworthy new features like built-in support for synchronizing bookmarks and other browser data.

Firefox 4 has been under development for over a year—the last major update, version 3.6, was released in January 2010. The 4.0 release arrives at a time when the Web is enjoying an unprecedented level of competition and a rapid pace of evolution. Although Mozilla arguably deserves a lot of credit for the role that it has played in accelerating the advancement of the open Web, the organization fell behind competing browser vendors due to the protracted length of its development cycle. The 4.0 release catapults Firefox back to the front of the pack, bringing parity in performance, features, usability, and support for Web standards.

User interface

Firefox 4 includes some of the most significant user interface changes in the browser's history. The tabs have been moved to the top of the window, above the main navigation bar. Other elements of the user interface have also been streamlined.

The stop and refresh features, which previously had their own dedicated buttons in the navigation toolbar, have been combined into a single button that is integrated into the tail end of the URL box. The bookmarks bar has been collapsed into a single button that is positioned at the right-hand edge of the main toolbar. The home button was moved to the immediate left of the new bookmarks button.

Mozilla has removed the conventional statusbar from the bottom of the window, replacing it with the same kind of transient statusbar that Google uses in Chrome. When the user hovers their cursor over the contents of an anchor tag in a Web page, the destination URL will appear in a floating rectangle at the bottom of the window. Mozilla experimented with several other ways of presenting the destination URL during the Firefox 4 development cycle, including an approach where the destination URL was displayed as an overlay in the URL box, before settling on Chrome's method.

The traditional menubar has been eliminated in the Windows version of Firefox, replaced by a single button embedded in the titlebar. The menu items have been greatly consolidated into a compact two-pane listing. Some items, such as copy and paste, have been integrated as buttons in the custom menu so that you don't have to dig into submenus in order to access them.

The add-on dialog box has been replaced by a new interface that opens in a tab. It feels much cleaner than the old dialog and has a much slicker design. In light of the fact that add-on users tend to use a lot of add-ons, giving the add-on interface more space seems like a really good move. Like the old dialog, it has a built-in search feature that allows the user to find new add-ons to install. The feature has been taken a step further in Firefox 4, however, because you can now see additional details about an add-on directly through the interface. It will largely obviate the need to visit the actual add-ons website for most users.