Mozilla has finally released Firefox 3.5 Beta, the latest version of its web browser that has been delayed several times already this year.

The release features more work under the hood - particularly in stability and performance of the new TraceMonkey JavaScript engine - as well as tweaks to the private browsing mode and improvements to the underlying Gecko layout engine, said Mike Beltzner, director of Firefox.

Tests by PC Advisor's sister website Computerworld US showed that Firefox 3.5 Beta 4 is about 19 percent faster than Beta 4 in the SunSpider JavaScript benchmark.

Other enhancements, Beltzner said in a message posted to the forum, have been made to Firefox's new location awareness feature. The browser now also supports web worker threads - an enhanced scripting functionality that lets site developers shift JavaScript computations to a background thread.

As it usually does, Mozilla warned users that Beta 4 is suitable for testing only.

"Firefox 3.5 Beta 4 is a public preview release intended for developer testing and community feedback," said Beltzner. We recommend that you read the release notes and known issues before installing this beta."

The beta's release notes highlighted several still-unfixed issues with Firefox 3.5, including problems with Gmail and with parts of the new privacy feature.

Beta 4 is the first major milestone of Mozilla's new browser to carry the '3.5' moniker; originally, the upgrade was called Firefox 3.1, but the company decided it had added enough new features to justify the larger bump in version number from last summer's Firefox 3.0.

Those additional features - especially TraceMonkey - caused some delays last year as Mozilla worked to both integrate the new JavaScript engine and test it. Other delays were due to troublesome bugs, many of them also in TraceMonkey.

Although Mozilla said much earlier in the Firefox 3.5 process that it would ship a limited number of betas before moving on to a 'release candidate', it's now unclear what Mozilla's next step will be.

In 2008, the company issued the last beta of Firefox 3.0 in early April, and the final version in mid-June. Assuming Firefox 3.5's pace is similar, Mozilla should ship Firefox 3.5 sometime in early July.

Firefox 3.5 Beta 4 can be downloaded for Windows, Mac OS X and Linux in 63 different languages from Mozilla's site. Users already running Beta 3 will be notified of the available update in the next 48 hours.