Mozilla posted the first beta version of Firefox 4 late Tuesday, offering users a first peek at its next web browser. Firefox 4 Beta 1 features a brand-new UI—on Windows Vista and Windows 7 only at this time—that adopts a more streamlined look and feel, as well as dozens of other new features. Mozilla says it will release new beta versions every two to three weeks and deliver the final Firefox 4 version by the end of 2010.

"Firefox 4 Beta 1 gives an early look at what's planned for Firefox 4," Mozilla Director of Firefox Mike Beltzner wrote in a blog post announcing the release. "Stay tuned, because there's more to come."

Those who install Firefox 4 Beta 1 on modern Windows versions will immediately notice the overhauled UI, which features a prominent orange Firefox button in the upper left, no visible menu bar, a reworked toolbar, and a Chrome-like row of tabs on the top edge of the browser window. And as with Chrome, the search box has been removed, adding to the streamlined look of the application.

Firefox 4 also features a crash-protection feature called OOPP (for "out of process plug-in") that prevents plug-in technologies such as Adobe Flash or Apple QuickTime from crashing the browser. The Add-on Manager has been made into a full-sized browser page (again, a la Chrome). It supports numerous HTML 5 features (a must-have in modern browsers), including WebM and HD web video. And there are various privacy-related enhancements, including fixing what Mozilla says are "flaws" in web standards.

Mozilla says that performance is a priority in this release. And although the Beta 1 release includes performance improvements at startup and during page loads, future releases will add other performance gains. Linux and Mac OS X users will also get the new UI in a future beta release, according to the company.

For Mozilla, Firefox 4 could well prove to be a pivotal release. Once considered the most prominent of the Internet Explorer (IE) alternatives, Firefox has stalled south of 25 percent usage share on a worldwide basis and hasn't moved the needle much in almost a year. (In fact, Firefox has actually lost share over the past six months.) The biggest problem for Firefox is the resurgence of Microsoft's browser; IE 8 has quickly become the most-often used browser worldwide and is, in fact, growing usage faster than any other browser version. But the new industry darling is Google's Chrome, which already looks and works much like Firefox 4. Chrome has been lauded for its performance and simple UI, and it appears to have taken usage growth away from Firefox, as well.

Readers interested in testing Firefox 4 Beta 1 can visit Mozilla's Firefox 4 Beta page.