Windows only: Reader David writes in about a user-contributed build of Firefox 3.5 Beta 4 that uses some custom patches and optimized build settings for faster, less memory-intensive browsing.
Multiple downloads are provided, each one optimized for the processor you are using—most modern PCs can use the P3 build, provided in both regular and portable downloads. The Flash and Windows Media plugins are already pre-installed, and the build uses the new profile-guided optimization option to speed it up even further. In our brief testing, it definitely feels fast and doesn’t appear to use much memory—but since this is a user-contributed, unofficial build by an unknown (to us, anyway) author, you may want to proceed with caution. Rest and Source