Internet Explorer 9 came out a few weeks ago, if you're not using it. I'd recommend it. Especially people who are still using Firefox. It is well documented as the fastest browser out there. Not just in javascript speed, but also important areas like start time, and page rendering.
With the update to Windows Phone coming later this year, Internet Explorer 9 will be included. With it comes all the HTML5, and hardware acceleration that IE9 brought to Windows, but now in your pocket.
Here's a video to give you an idea:
To sum up, Internet Explorer 9 on Windows Phone renders at 23 fps, Chrome comes in at 11 fps on Android, and Safari on an iPhone 4 comes in at a dismal 2 fps.