It seems that most browsers are focused on speeding up javascript execution now as most websites are heavy with ajax and complex js.
With Firefox releasing their pre-beta "dev" version Aurora this morning I decided to run all the current browsers through sunspider to see if any progress has been made.
Chrome-dev: 294ms
Aurora (FF dev): 324ms
Firefox 4: 348ms
IE9: 251ms
IE10 preview 1: 254ms
So it would seem that Firefox has sped up their js engine, but has a little way to go to catch up to the other browsers.