Waterfox provides you with a high-performance Open Source 64-Bit version of Mozilla Firefox. Waterfox Portable is also available.
The Firefox source code was taken and compiled to run for 64-Bit Windows computers. It is not a flashy or elaborate browser by any stretch, but it will import all of your Firefox settings and incorporate them, making it very familiar to use.
Waterfox is compiled with many optimizations to perform faster and more efficiently than merely redistributing Firefox as a 64-Bit program.