Benoit Girard recently wrote about the cross-platform profiler he's working on implementing into Firefox. The post contains a great introduction to instrumented and sampling profilers, for the ones so inclined.

Personally, I think that this is a huge step forward for web development especially as we move more and more work client side where performance in the browser really starts mattering. In 2010 I wrote a piece of photo editing software only using JavaScript and HTML, and back then it sucked trying to find the slow bits of your code or even having a reasonable way to compare performance of two iterations. I'm really looking forward to this ending up in GA, so that we can get SeƱor Foxie out in front!