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Computerworld - Mozilla today released Firefox 3.6, the first upgrade to the open-source browser since last June.

Firefox 3.6 was posted to Mozilla's download servers early Thursday, just over five months after the browser first entered public testing with an alpha release. The upgrade shifted to beta in early November 2009, then wrapped up with a pair of release candidates (RC).

RC2, which reached testers last weekend, was sufficiently bug-free that Mozilla renamed that build as the final it shipped today.

The new browser includes built-in support for the scaled-down browser skins dubbed "Personas;" adds support for new CSS, DOM and HTML 5 technologies, as well as for full-screen video and the open-source Web Open Font Format (WOFF).

Firefox 3.6 also boasts a new security feature that checks browser plug-ins, such as Adobe's Flash Player or Apple's QuickTime, to make sure they're up-to-date, then blocks vulnerable plug-ins from loading and shows users how to update the software. Mozilla added some plug-in checking to older editions starting last September, but fleshed it out in Firefox 3.6.

Mozilla has also boosted the browser's overall performance and reduced its startup time. JavaScript in 3.6 executes approximately 12% faster in Firefox 3.6 than in 3.5, said Mike Beltzner, the director of Firefox, while some pages load up to 20% faster. Other improvements that speed up location bar searches and let Web site designers use asynchronous script loading, are harder to measure, but should give users the feeling that Firefox 3.6 is snappier. "

We did a lot of work with Web developers and with our own code," said Beltzner, "so users will feel the speed."

Mozilla is touting the update with the line, "World's best browser" on its site, an assertion that's likely to raise hackles from the competition. "I believe it's true," said Beltzner, defending the claim. "Firefox lets users take control of their experience, it's the most customizable browser around, and it's an incredibly fast browser. So, from the overall experience [perspective], we think Firefox has the best complete package."

Firefox 3.6 landed about two months later than initial estimates -- last fall, Mozilla said it was shooting for a November delivery of the final -- but the delay was much shorter than the half year the company had to add to the development schedule of Firefox 3.5.

Mozilla has made other changes to its post-3.6 plans. At one time, today's release was to be the first of two minor updates that Mozilla would deliver over a six-month stretch, part of a plan to speed up development and stay feature-competitive with hard-charging rivals such as Google's Chrome. Recently, however, Mozilla has decided to drop the second of those minor updates and instead roll selected features into Firefox 3.6 as part of the year's security updates. Meanwhile, the next major update will go through the standard public testing cycle, and should ship in late 2010 or early 2011.

The appearance of Firefox 3.6 also starts the end-of-life clock ticking for Firefox 3.5; Mozilla's policy is to support an older edition for approximately six months after the launch of a successor, meaning the kill date for 3.5 will likely be in late July 2010. At or around that time, Mozilla will stop producing security patches for the older browser.

Firefox 3.6 can be downloaded in Windows, Mac and Linux editions in 65 languages from Mozilla's site; current users can update by choosing "Check for Updates" under the "Help" menu.

Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer, send e-mail to gkeizer@ix.netcom.com or subscribe to Gregg's RSS feed .

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