The second beta release of the next version of Firefox is now available.

Download Firefox 4 Beta 2 from Mozilla and test it out. Windows, Mac OS X and Linux builds are available in multiple languages. We were originally expecting it to arrive last Friday, but the release was delayed a few days for quality assurance testing.

Keep in mind, this is a pre-release version of the browser, and it may not be entirely stable. But it should be stable enough for daily use, and it will give you a heads up on all the new goodies coming in Firefox 4 when it's officially released this fall.

Tuesday's release has a number of new features, including support for CSS 3 transitions, better handling of retained layers on pages and a new feature in the add-ons manager that confirms when an add-on has been installed. There are also the requisite performance boosts and stability improvements, so if you're running beta 1, definitely consider upgrading.

The feature sure to generate the most chatter is something new for Mac OS X users: a new tabs-on-top interface. Windows users got the tabs-on-top look as the default interface in Beta 1 earlier this month. With beta 2, the change arrives on Macs. The new beta also enables App Tabs, a similar concept that lets you miniaturize the tabs for common web apps – e-mail, your calendar, or other apps you use multiple times a day – and store them in the tab bar for quick access.

The move to tabs-on-top is a growing trend among browser vendors. It was popularized by Google Chrome, which has shipped with top-tabs as the default since its birth two years ago. Reaction has been mixed – Opera now puts the tabs on top, and Safari tried the same thing in a beta release thing before abandoning it. And there are some within the Firefox user community who fear Mozilla is making the switch just to chase the latest design fad.

Mozilla's lead user experience designer Alex Faaborg defends the decision, saying it has nothing to do with fashion. By putting the tabs on top, he says, Firefox 4 will be better equipped to run web applications that sit in their own tab.

These UI tweaks turn the tab bar into something much closer to a dock or a task bar – a fitting change, since the browser is becoming something much closer to a GUI for an operating system. Of course, if you don't like your tabs up top, you can always choose the old look in the browser's View menu.

The final browser is expected in October or November, and you can read our preview of Firefox 4 on Webmonkey.

This article originally appeared on Webmonkey.com, Wired's site for all things web development, browsers, and web apps. For more from Webmonkey, follow the links at the end of the article.

Illustration at the top courtesy of Mozilla.

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