Firefox just released the first beta of Firefox 18, which will be a milestone release for Mozilla as it introduces the IonMonkey JIT compiler.
Besides IonMonkey, Firefox 18 will not deliver much else. According to Mozilla's feature tracking, the browser will get per-site third-party cookie setting support as well as inline PDF support. The final version of the browser is scheduled for release on January 1, 2013.
The beta version can be downloaded here.