Mozilla has released Firefox 9, which brings speed improvements and uses less memory than previous releases. In fact, this release effectively puts Firefox back on a level playing field with Google Chrome when it comes to speed.

If you’d like to try out Firefox 9, head on over to the Mozilla downloads page. If you’re already using Firefox you’ll be automatically updated to version 9.

The big news in this release is under the hood where Firefox now supports what’s known as Type Inference. Type Inference is a new feature for Firefox’s SpiderMonkey JavaScript engine and means that complex JavaScript websites — which, let’s face it, is pretty much every website these days — should run faster. According to Mozilla, Firefox 9’s Type Inference should make the browser between 20 and 30 percent faster.

Alongside the faster JavaScript processing Firefox 9 continues to show improvements from Mozilla’s MemShrink project, an ongoing effort to reduce memory usage in the browser. Indeed, for the first time in a very long time my testing showed Firefox 9 using less memory than Opera (which has long been the least RAM hungry browser I test). Opening the same dozen tabs in both Firefox and Opera used only 367 MB of RAM in Firefox compared to 378 MB in Opera 11.60. There’s no longer much difference between the two, which is a testament to Firefox’s dramatic improvement over the last six months of MemShrink efforts.

Web developers get a few new toys in this release, including a fullscreen mode that allows any HTML element to take over the screen. Although fullscreen is primarily associated with video elements, there may be occasions (for example, HTML elements used in web-based games) where it makes sense to take over the screen. For now the fullscreen feature needs the -moz prefix to work.

Firefox 9 also includes a new “dim the lights” feature for HTML5 video. Dimming the lights means that Firefox will overlay the rest of the browser window with a gray background that let’s you focus on the video in question. Check out this demo video which shows the dimming in action.

While most of what’s new in Firefox 9 is under the hood, Mac users will notice a few cosmetic changes like a slightly tweaked look and feel that more closely matches the Mac OS X Lion toolbar styles. There’s also now support for two-finger swipe gestures to navigate back and forth in history (mirroring the same features in Chrome and Safari).

Firefox 9 is well worth the upgrade. If you moved away from Firefox due to speed problems and bloat this release warrants another look. Those plagued by the rapid release cycle’s habit of breaking add-ons may want to hold off though. Firefox 9, for all its other improvements, may still break some add-ons. Mozilla has a solution to the breaking add-ons problem in the works, but it won’t arrive for another six weeks when Firefox 10 is released.