Swiftfox is optimized Firefox. It comes with many editions for your CPU. I downloaded it for my Athlon 64X2 box. It is loading and surfing very fast. I recommend all of you to use it.
My Firefox was left behind on the panel without touching since then.
Updated on 2007-06-21 :
I installed Fasterfox addon as advised by huygens-25. It is running smooth and fast. It also blocked the popup flash in some websites. A real good stuff.
Samiux's Blog 
Swiftfox, as compared to Firefox, changes only a few default values (which an extension like Fasterfox corrects too) and is compiled with optimisation flags and a dedicated architecture.
So if you compare Firefox+Fasterfox with Swiftfox, the only difference is the compilation (and a few artworks and brand name).
In my experience, I really never felt much different between an optimised compilation and a standard one. People making benchmark finds a 2-5% better performance, something an end-user would not feel. A really optimised Gentoo installation is also not that faster than a Debian installation (at similar kernel and software version).
Furthermore, when you download Swiftfox, you are downloading a binary, which you have to trust. You do not know if other changes than those reported by the author have been done. A safe way, is to get the Firefox source code, use what the author of Swiftfox advertise for compilation flags and compile the source. Then, install Fasterfox extension and the result will be a Swiftfox equivalent that one can trust.