I stumbled this morning on a great quote from Edwin Khodabakchian, who develops Feeddo/Feedly, posted on Scoble's blog :

We did some benchmarking between firefox 2.0 and firefox 3.0 on one of our applications which uses heavily Javascript, DHTML and timers and the results are very impressive: a much more fluid experience thanks to the timer/scheduling experience and an overall execution performance which is 3.5x faster. I am not exactly sure how they did it but the results are impressive.

Disclaimer: Edwin is an old friend of mine, when we were both Netscape employees @ the Paris office. But I promise I've found his quote serendipitously!