App Tabs let users pin frequently-visited sites to the left-side of the tab bar.
App Tabs let users pin frequently visited sites to the left side of the tab bar. (Credit: Screenshot by Seth Rosenblatt/CNET)

Mozilla's second Firefox 4 beta debuted on Tuesday, with interface and feature improvements for Windows and Mac users, and under-the-hood changes that include faster browser launch times. Available for Windows, Mac, and Linux, Firefox 4 beta 2 doesn't make any radical visual changes to the browser, but it does introduce a couple of new features and support for new developer tools.

The only interface change made this time around was to give Mac users the tabs on top look by default. Windows users received that feature in the first beta. This can still be reverted under Preferences. Linux users have yet to receive tabs on top by default.

One of the big features that some users have been anticipating in Firefox 4 is App Tabs, analogous to Google Chrome's Pin Tab feature. It reduces a tab to its favicon, and then anchors it permanently on the left side of the tab bar. This can make it easier to find regularly frequented sites, such as Web mail or a calendar. To create an App Tab, right-click on a tab and choose, "Make into an App Tab." One of the principal designers on Firefox, Alex Faaborg, has created a video that details how to use the App Tabs. He also notes that App Tabs will receive some kind of special treatment in the upcoming Tab Candy feature.

How Firefox 4 functions and handles the code that powers the Web has changed slightly, too. The biggest change, made to the XPCOM registration, has the potential to break a lot of components that interact with Firefox. While that could result in a lot of headaches over the near future, users should see faster Firefox start-up times in the long run.

Meanwhile, part of the CSS3 Transformations, called Transitions, are now supported in Firefox 4. This means that developers will be able to build content that animates instead of effecting an immediate change. Curious readers can check out the yellow box demo on the Mozilla Hacks blog. Support for retained layers also shipped in the second beta, which allows for faster page scrolling.

Mozilla also noted that user comments sent via the "feedback" button on the right-hand side of the navigation bar helped the publisher fix more than 670 issues in the first beta. Future betas promise to include new features such as the aforementioned Tab Candy and the redesigned add-on framework, currently code-named JetPack, but also to resolve unsettled debates such as whether to remove the Status Bar and the struggle to improve the browser's overall site-rendering speed.