Firefox 9.0 Beta for Windows, Mac and Linux is now available for download and testing. Like any other version it comes with a lot of new and improved features.

The most valuable update is a big boost to JavaScript performance. The enhanced performance comes through the release of Type Inference, or TI, a research project under way for over a year. TI is a feature in the SpiderMonkey Javascript engine which generates type information about Javascript programs through a combination of analyzing the program’s code and monitoring the types of values as the program executes. This type information is used during JIT compilation to generate more efficient code; Firefox 9 includes modifications to the JaegerMonkey JIT compiler to use inferred type information. This compilation mode, which is the default in Firefox 9, speeds up major benchmarks like Kraken and V8 by over 30%, and gives a large speed boost to many JS heavy websites.

What’s New in Firefox 9.0:

• Type Inference: Firefox Beta adds Type Inference to make JavaScript significantly faster
• Mac OS X Lion Support: Firefox Beta has a new look that matches the Mac OS X Lion application toolbar and icon styles. Firefox Beta also supports the two finger swipe navigation gesture and makes it easier to use multiple monitors.
• Do Not Track JavaScript Detection: Firefox Beta enables JavaScript to show developers when users choose to opt-out of behavioral tracking with the Do Not Track privacy feature.
• Chunked XHR Support: Firefox Beta supports chunking for XHR requests so websites can receive data that’s part of a large XHR download in progress. This helps developers make websites and Web apps faster, especially those that download large sets of data or via AJAX.

The full list of release notes can be found here.