Firefox 18 final hasn't been officially released yet but you can download it from Mozilla's server. Mozilla can make the official announcement at any moment today. Released officially! Check for updates from the file menu or download from the link below.
Firefox 18 includes a faster JavaScript engine 'IonMonkey' to improve page loading times by up to 26% over the previous version, according to Mozilla's own benchmark. On Google's V8 benchmark, Firefox 18 is 7% faster than Firefox 17.
Touchscreens support has also been improved, with Mozilla now implementing W3C touch events in place of its own proprietary MozTouch standard. Mac OS X users get full retina display support with this version.
Download Firefox 18 for Windows
For Linux and Mac OS X version click here.
Update: Release Notes