Firefox 9 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.
Full Release Notes: http://www.mozilla.org/en-US/firefox/9.0/releasenotes/