Mozilla has unleashed Firefox 4, the next version of the popular open source web browser.

The fourth major upgrade for Firefox was originally scheduled for the end of 2010, but the sheer number of new features pushed the deadline back several times. Now the wait is over — the new and improved Firefox 4 is now available as a free download for Windows, Mac and Linux.

Firefox 4 is a major overhaul, bringing a revamped user interface, hardware accelerated-speed improvements, built-in syncing and a significant upgrade under the hood. Page-rendering speed has been tripled, according to Mozilla, and the browser has increased support for web standards like native HTML5 video, fancy CSS 3 visual effects and native web fonts.

User interface

The most noticeable change in Firefox 4 is the revamped user interface, which streamlines the look and feel of Firefox. Tabs have been moved to the top of the browser window, above the URL bar. On Windows Firefox 4 matches Chrome’s tightly stacked tabs (which extend into the window’s title bar), and it does reduce the amount of space the tab bar takes up. On Mac OS X Firefox 4’s tabs don’t extend into the title bar. If you don’t like the tabs on top, there’s a preference option to revert to the old look.

Tabs are on top in the new Firefox 4.

The status bar, which has long lived at the bottom of the window, is now gone. Instead there’s an “Add-ons bar,” though it’s disabled by default. The main missing information in the status bar — the URL preview that shows up with you hover over a link — now shows up in a floating window, a la Google Chrome and IE 9.

Also gone in Firefox 4 is the RSS button, which, according to Mozilla’s research, no one was using. If you were among the few who did use the RSS button, it’s not hard to add it back.

Clearly Chrome’s minimalist user interface has inspired Firefox to clean up its look (Chrome had a similar effect on the recent release of Internet Explorer 9), but Firefox still has a few tricks up its sleeve.

Firefox 4 introduces a new feature dubbed “Panorama.” With it, you’ll be able to group and quickly switch between related clusters of open tabs. Designed for those of us over-stimulated fiends who frequently have dozens of tabs (or more) open at one time, Panorama allows you to conquer tab chaos. For example, you can group tabs for work and tabs for fun, and then quickly switch between groups.

App tabs save space in the tab bar.

This release brings another new feature, App Tabs, which allow you to pin sites you use frequently — Gmail or Facebook, for example — to persistent tabs that take up less space and will stay in place even when you switch between tab groups.

There are so many new features in Firefox 4 we don’t have the space to list them all here, but a few standouts in the new interface include the ability to switch to a new tab when you search in the URL bar, the slick new add-ons manager and privacy controls that stop websites from tracking your every move.

Syncing

Most of us use multiple screens every day — one or two computers, and at least one smartphone with a web browser — keeping it all in sync is increasingly difficult. That’s where Firefox 4’s sync tools come in, allowing you to pick up where you left off, no matter what device you’re using.

Firefox 4’s sync feature handles bookmarks, browsing history, user preferences and open tabs, allowing you to move between desktop and mobile versions of Firefox with all your data intact. Firefox Sync is simple to use: Just create a username and password, along with an encryption phrase, and Firefox takes care of the rest behind the scenes.

Unlike similar implementations in other browsers, Firefox 4 will encrypt all your data before sending it over the network to sync through Mozilla’s servers, which ensures that your data is protected from prying eyes (you can also host your own sync server if you’d like).

The sync updates in Firefox 4 coincide with similar improvements in Firefox Mobile 4 for the Android and Maemo mobile platforms. For iOS users there’s the Firefox Home app, which can give you access to all of your synced info. (Though any links you open will of course be rendered in iOS’ Webkit browser, not Firefox.)

The one missing feature in Firefox 4’s syncing is add-ons. As Firefox transitions to its new add-on format, it’s possible future releases will sync add-ons, as well.

Speed and hardware acceleration

Firefox 4 makes impressive speed gains (note tests show RC1)

There wouldn’t be much point to upgrading your web browser if the latest version wasn’t faster. And when it comes to speed, Firefox 4 delivers. Thanks to improvements in the Gecko rendering engine and the JavaScript engine, Firefox 4 is significantly faster than its predecessors.

This release also sees the first support for hardware acceleration. The idea behind hardware acceleration is to hand off processor-intensive tasks to the computer’s graphics card so that animations and page rendering are faster and smoother. Hardware acceleration is particularly helpful for common tasks like rendering text and graphics.

Internet Explorer 9 hyped its hardware acceleration when it launched last week, but Firefox 4 not only bests IE 9 in several tests, it also supports Windows XP, while IE 9’s acceleration is limited to Windows Vista and Windows 7.

In fact Firefox’s hardware acceleration works across platforms taking advantage of Direct2D and Direct3D on Windows, XRender on Linux and OpenGL on Mac, to make animations and complex HTML5 applications run more smoothly. Firefox 4’s hardware acceleration is enabled by default on all supported hardware.

Web standards

Mozilla's Web o' Wonder shows off Firefox 4's HTML5 powers

Firefox has long been at the forefront of standards support, and this release is no exception. Firefox 4 brings support for HTML5 features like the audio and video tags (rendering native web video with the WebM and OGG codecs), as well as the new semantic elements and dozens of APIs, ranging from drag-and-drop file uploading to geolocation.

You’ll also find plenty of support for new CSS 3 features like transforms, transitions, media queries for responsive design and @font-face for embedding better-looking fonts in your web pages. To go along with the new font embedding, Firefox 4 now supports the Web Open Font Format (WOFF), which joins the TrueType and OpenType support found in previous releases.

Firefox 4 also drops the -moz prefix for a number of more stable CSS rules, like border-radius and box-shadow.

Firefox 4 now supports WebGL, which means faster 3-D graphics and animations. WebGL bridges the gap between HTML5 tools like the new Canvas tag and OpenGL, an OS-native graphics engine, to speed up HTML5 web apps and animations.

To see the new HTML5, CSS 3 and API support in action, head over to Mozilla’s Web o’ Wonders demo page which showcases the new standards support in Firefox 4.

Conclusion

Firefox 4 is a massive overhaul and you should definitely upgrade. Not only is Firefox 4 faster, it brings the modern web to your desktop thanks to excellent web standards support.