The latest version of Mozilla’s Firefox has returned to the theme of improved performance, boosting JavaScript speed by a claimed 30 percent plus on websites making extensive use of it.

It’s barely six weeks since the world was handed Firefox 8 on the company’s rapid-release schedule, but version 9, which has just reached beta prior to release next week, continues the browser’s evolution with a number of mostly hidden improvements.

The main enhancement this time is the addition of Type Inference technology to Firefox’s SpiderMonkey Javascript engine, basically a way of automatically optimising code fed to the JIT compiler.

JavaScript-heavy sites should see a significant benefit, although we found after benchmarking with sunspider that it barely reached a 10 percent improvement over version 8; the most expensive but spectacular way to improve computationally-intensive activities such as JavaScript on a PC is still to buy a more powerful CPU.

Most of the other improvements are marginal except for Apple users who get better support for themes and two-fingered swipe navigation for OS X Lion.

On the JavaScript theme, developers now have the ability to query the Do Not Track status, plus improved support for HTML5, CSS and MathML.

Version 9 arrives at a time of considerable challenge for a browser once rated the techie’s clear favourite. Only last week, StatCounter declared that Google’s Chrome had become the number two browser behind IE, a position that has long been Firefox's.

A Google-sponsored survey by Accuvant also recently handed the browser security crown to Chrome thanks in part to its 'hardened' JavaScript JIT compiler.

Chrome’s pre-eminence is probably inevitable given Google’s determination to promote it to the large number of users of its Gmail service for one. Interestingly, Google is also a heavy user of JavaScript on its websites.