Mozilla has released Firefox 3.1 Beta 2, the first version of its flagship browser to turn on a much faster JavaScript engine and sport a working privacy mode.

Following the first beta by about eight weeks, the newest preview switches on TraceMonkey, the JavaScript engine Mozilla initially touted back in August. Beta 1 included the new engine, but it had been disabled by default; users were required to manually edit the browser's configuration file to turn on the engine.

Also present in Beta 2 is "Private Browsing," the privacy mode that Mozilla decided to add in September, but didn't make add to a test build until early last month.

Private Browsing was a reaction to similar additions in other browsers, including Apple's Safari, Google's Chrome and Microsoft's Internet Explorer 8. Privacy mode lets users surf without leaving obvious traces of where they've been, which has led to the feature being dubbed "porn mode" in a nod to one of its more obvious uses.

An ancillary addition to Private Browser is a new addition to the "Clear Recent History" dialog box that allows users to selectively erase the last hour, the last two hours, the last four hours, today's or all browsing history. Previously, the wipe was all or nothing.

Firefox 3.1 Beta 2 includes support for "web worker threads," a developing specification that will let Web-based application developers run background processes to speed up their apps.

One feature trumpeted by Mozilla in Beta 1 has disappeared in the newest version. As Mozilla noted in late November, it pulled a revamped Ctrl-Tab tab-switching feature because it wasn't satisfied with the tool's user interface. The now-absent Ctrl-Tab redesign, which was based on an existing add-on of the same name, showed users thumbnails when they cycled through open tabs, and switched between current and last-viewed tabs rather than simply moving to the next tab on the right.