[kevincennis]

This is cool, but Firefox still feels a lot slower to me than Chrome.

I think Mozilla would get a lot of mileage out of improving the performance of the rendering engine. FF doesn't feel slow because of JS, it feels slow(-er than Chrome, at least on my Mac) because repaints and layout seem to take longer.

[agapos]

They are working on it on several fronts, constantly cutting off tasks from the main thread onto their own ones (downloading, cache v2, Awesomebar), making other things asynchronous (project snappy), garbage collection improvements also help in some cases. Most of these are smaller wins, and several big ones like OMTC, tiled rendering or e10s are yet to come.

Edit: It may not be as responsive as Chrome (I don't know as I don't use it), but I believe it is miles ahead of it's former self.