Well, after a long time playing with nightly Webkit because Firefox wasn't so good on my mac, I decided to give it a try again with 3.1 beta2 and.... Wow! It's amazing! Super fast and I can even say that it seems faster than last webkit builds. I also liked that it seems to use a little less cpu than webkit which is very nice!
Kudos to Mozilla Firefox people who is doing an excellent job regarding performance of firefox on OSX and from what I saw on other OS's too.