Firefox-developer The Mozilla Foundation says it's going to release Beta 4 of Firefox 3.5 sometime later this week despite the lack of a firm release date. Beta 4 will likely be the last test version before Mozilla moves on to the release candidate of Firefox 3.5. However, Mozilla has not made good on many of Firefox 3.5's deadlines: earlier this year, Mozilla has already delayed the release of Beta 3 -- twice -- before moving on to Beta 4, as well as dumping the name Firefox 3.1 for the snappier Firefox 3.5 to reflect the significant differences between versions 3.0 and 3.5.

Most of the previous delays stemmed from Firefox's new JavaScript engine, TraceMonkey, which promises to speed up page loads two times faster than Firefox 3.0 and nine times faster than Firefox 2.0. At the moment, Beta 4 contains seven high-priority bugs requiring fixes, five of which involve the JavaScript engine.

When it is finally ready for mass adoption, Firefox 3.5 promises some new features largely based on HTML 5 -- the latest update to the core language of the World Wide Web. Mozilla says Firefox 3.5 will deliver noticeably improved performance, security and a better user experience overall. More specific features include:

While Mozilla works to resolve problems with Firefox 3.5, the foundation is also hard at work on Firefox 3.0. Today, Mozilla released Firefox 3.0.9 update with 12 security fixes -- four of which were critical. Today's update was the largest security patch for Firefox since December 2008, but the second security update in the last four weeks.

In late March, Mozilla released a fix for the so-called 'zero day' flaw after a German hacker exploited the browser to help him win US$15,000 during the Pwn2Own competition at the CanSecWest conference.

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