[sayrer]

It's actually around 10% faster at a bunch of things. :) This includes some DOM APIs and just raw JS perf, depending on your platform.

Responsiveness will be improved in a different manner. We are working on a bunch of improvements for later in 2011 that should make a big difference. I think Firefox 4 is in general much better than 3.6, and I expect to see parallelism, responsiveness, and reliability continue to improve.

The big ticket items are multiprocess browsing and better debugging APIs--we have an issue with developer extensions causing performance problems. This is not necessarily the fault of Firebug or any of the other extensions. We need a better foundation for them to build on, and it really hurts us to give developers a slow experience.