Mozilla on Tuesday released the first official beta of Firefox 4, the next major version of the popular open source Web browser. Mozilla has completely overhauled Firefox's user interface and added several noteworthy new features for Web developers and regular end users.

The new user interface represents a major departure from Firefox's traditional look and feel. It is arguably one of the most significant stylistic overhauls that Mozilla has undertaken since the initial transition from the old Mozilla suite to Firefox.

We tested the beta on Windows 7 because that is the platform on which the new theme is most mature; design work is still ongoing for Mac OS X and Linux. The new Windows theme is intended to match Microsoft's glass aesthetic. The tab bar was moved to the top of the window where it melds seamlessly with the titlebar. The conventional menu was stripped out and replaced with an orange lozenge that is mounted in the top left-hand corner of the titlebar. The entire title region and the inactive tabs are semi-transparent. The inactive tabs are slightly frosted for readability.

The new main menu leaves out a lot of the standard items that were previously accessible through the conventional menu bar. Many of the omitted items are still easy to reach through contextual menus and other parts of the user interface, but there are several features—such as private browsing and the download window—that are no longer easily discoverable. You can still use those features by turning back on the classic menu or hitting Alt to temporarily activate it. It's likely that these features will be exposed in more practical ways as the design work progresses.

The bookmark list now has its own dedicated menu button located in the main toolbar. When you enable the bookmark toolbar, the bookmark button gets sucked down into it and will disappear from the main toolbar. It's a particularly nice touch and a good use of space.

The new design looks promising, but it still has some rough edges and needs further refinement. There are still some parts that are awkward and feel out of place. If you look at the right and left sides, for example, you can see that the elements don't really line up properly. The way that the tabs run up against the window titlebar buttons on the right-hand side also looks wrong to us, like it needs a little bit more vertical spacing.

Mozilla has replaced the old add-on manager window with a new interface that lives in an actual tab. This seems like a good move in light of the growing complexity of the add-on manager. The new add-on interface feels less cramped and cluttered due to the significant amount of spacing. It seems likely that Mozilla will integrate more functionality from its add-ons website into the browser's built-in add-on user interface in the future.

Mozilla's support for emerging Web standards continues to advance in the new version of the browser. Google's open WebM video format works out of the box in Firefox 4, supported alongside Ogg Theora in the HTML 5 video element. We tested it by viewing several WebM video samples in YouTube's HTML5 player. Another major addition in Firefox 4 is support for WebGL, an open standard for displaying 3D content in webpages. Although WebGL is implemented, it's not enabled yet in the beta release.

On the roadmap

There are several additional features not included in the beta that Mozilla plans to add under the hood prior to the release of Firefox 4. We can expect to see these features arrive soon, because Mozilla is moving forward quickly with an aggressive development schedule.

One of these features, JetPack, a lightweight extension system based on HTML and JavaScript, is going to be integrated directly into the browser and will be supported alongside conventional add-ons. JetPack was initially introduced by Mozilla Labs last year. Much like the Persona feature that was added in Firefox 3.6, JetPack was initially developed as an add-on itself so that it could mature independently before being adopted as a standard part of Firefox. It's a nice feature that will help lower the barrier to entry for extending the browser.

Mozilla was arguably the industry leader when Firefox 3 was released in 2008, but other browser vendors have jumped ahead by delivering superior performance and reliability. Mozilla is now in a position where it has to focus on becoming more nimble if it wants to retain its relevance in the increasingly competitive browser market. The new features in Firefox 4 are a promising step in the right direction.

Users who want to try out the new beta release can download it from the Firefox Web site. For additional details about the new version, you can refer to the release notes or the official release announcement in the Mozilla blog.