I don't think it has been improved at all in start-up speed, or any other speed aspects. I still sometimes get a spinning beach ball when I start it for the first time. Besides, sometimes when I right-click it to close it, it might not close right away, and you can hear the HD grinding, which is annoying. I still find it "bulky" as opposed to the light and speedy Safari. Also, at times when I want to type a web address, it will lag for a couple of secs to actually start displaying the letters I typed. Or sometimes when I type a letter, it will lag for a couple of secs to display all the links that start with that letter (you know what I mean, the "associated sites" search..). It's a shame because I love FF, yet it's for the speed reasons that I now got sick of it and pretty much switched to Safari 4.