Firefox 9 has winged its way to various mirrors across the web and is now available to download from the official Firefox website.

The most significant change over Firefox 8 is the addition of type inference to the JavaScript engine, which singlehandedly improves JS execution speed by 20 to 30%. Without getting into the complexities of this change, type inference basically brings JavaScript one step closer to compiled languages (like C or C++) in terms of speed and optimization. For more information, see our type inference explainer. As far as we’re aware, Internet Explorer, Chrome, and Safari all lack this rather juicy feature. It’s a bit too early to say that Firefox now has the fastest JavaScript performance of the Big Four, but it’s definitely very close.

If you’ve been using either IE or Chrome because of their superior JavaScript performance, now would be the time to give Firefox another spin; you’ll be surprised at how well it deals with JavaScript-heavy sites, like Gmail, Facebook, and Google+.

Along with a bunch of bug and stability fixes, and overall speed increases, the Mac OS X version of Firefox 9 now allows for “two finger swipe navigation,” (after playing with Firefox 9 on Mac for half an hour, we’re not sure what this entails — leave a comment if you find out) and deeper theme integration (presumably some themes couldn’t skin some parts of the Mac UI, but they can now). Finally, there’s some additional HTML5 and CSS support, and new hooks that allow developers to query your Do Not Track status via JavaScript.

Download Firefox 9 now (Windows / Mac / Linux) or see the full release notes