Firefox 9 incorporates a new JavaScript engine that promises another 20-30 percent performance boost thanks to type inference support. This feature analyzes JavaScript before compiling it in order to process the code more efficiently. Here's a list of all the major changes in this new release:
Added Type Inference, significantly improving JavaScript performance Improved theme integration for Mac OS X Lion Added two finger swipe navigation for Mac OS X Lion Added support for querying Do Not Track status via JavaScript Added support for font-stretch Improved support for text-overflow Improved standards support for HTML5, MathML, and CSS Fixed several stability issues Fixed several security issues