im not sure about it neither, probably the browser really can use the advantage of modern cpu instruction set, im not very much into compilation; but ive tested it by myself and there truly is difference - same as you can see on the website, ive been using palemoon until the firefox 4 betas came out...and i will use it again when its ready for ff4

Browser just like any other app can take advantage of advanced CPU instructions if it's compiled for a specific architecture. For example, I know for sure that there is pretty significant performance increase in Avidemux (think linux VirtualDub) when I compile it with "-march=core2" instead of "-march=x86_64" in GCC. What I remember from the times I was running on Gentoo, FF compiled with a boatload of optimizations in GCC wasn't any faster than an official binary build, I guess it's because browser isn't all that CPU intensive for these optimizations to have effect.

Quote from MadCatX :

Browser just like any other app can take advantage of advanced CPU instructions if it's compiled for a specific architecture. For example, I know for sure that there is pretty significant performance increase in Avidemux (think linux VirtualDub) when I compile it with "-march=core2" instead of "-march=x86_64" in GCC. What I remember from the times I was running on Gentoo, FF compiled with a boatload of optimizations in GCC wasn't any faster than an official binary build, I guess it's because browser isn't all that CPU intensive for these optimizations to have effect.

i've always wondered how much xorg plays into firefox's performance...

as for core2 vs the generic x86-64, i'm afraid i don't know what gcc options that expands to, so i can't say much there.