Mozilla has finally released Firefox 3.1 Beta 3, the long-delayed update that the company claims offers improvements to the new JavaScript engine and private browsing tools.

Two weeks ago, Mozilla executives promised to move forward on the stalled Beta 3 even if developers weren't able to patch a troublesome bug in the new TraceMonkey JavaScript engine. To compensate, they added a fourth beta to the schedule. Beta 4 is currently planned for release 14 April.

According to Mozilla's Bugzilla problem-tracking database, the TraceMonkey flaw that held up Beta 3 has not yet been fixed.

As it always does, Mozilla warned users that the beta is a preliminary version for testing purposes. "Firefox 3.1 Beta 3 is a public preview release intended for developer testing and community feedback," Mike Beltzner, director of Firefox, said in an entry to the company's developer blog. On the download page, it got even more specific: "This is pre-release software: you may encounter compatibility problems with some websites and add-ons."

Of the extensions that make up 95% of add-on usage, just 45% are currently up-to-date with the latest version of Firefox 3.1, while another 34% are compatible with an earlier beta version of 3.1, according to Mozilla.
Beta 3 features improvements to Firefox's "Private Browsing Mode," a set of tools that debuted in December 2008 when Mozilla issued Beta 2. Now, however, users can selectively tell the browser to "forget" specific already-visited Web sites from the History sidebar by right-clicking and choosing "Forget About This Site."
Beltzner also touted stability and performance improvements to TraceMonkey, faster rendering of Web content by virtue of speed tweaks to Firefox's underlying Gecko layout engine, and changes to what Mozilla calls "Web worker threads," an enhanced scripting functionality that lets site developers shift JavaScript computations to a background thread.

Beta 3 is the last public preview that will carry the Firefox 3.1 label, as earlier this week Mozilla decided to dump that name in favor of Firefox 3.5. The name change had been under discussion for several weeks, prompted in part by calls from developers who thought that 3.1 didn't properly reflect the amount of new features and changes from last June's Firefox 3.0.
Mozilla conceived Firefox 3.1 as a "fast track" upgrade that it originally intended to launch in late 2008. But the browser's progress has been slower than planned, and the company has reworked its schedule several times.

Last November, for example, Mozilla added Beta 3 to the timetable to give developers more time to fix bugs and test features such as TraceMonkey.  This year, Mozilla first pushed back Beta 3's release in mid-January, then a week later again delayed delivery. Mozilla has declined to set a final delivery date for Firefox 3.5.

Firefox 3.1 Beta 3 can be downloaded for Windows, Mac OS X and Linux in 59 different languages from Mozilla's site. Users already running Beta 2 will be notified of the available update in the next 48 hours.