There is no official 64 bit version of Firefox for Windows. Current release versions are 32 bit, and Firefox 4 on Windows will be 32 bit as well.

There are development versions being tested that are 64 bit but they are only recommended for testers and not general use.

but i think its better for me to use 64 bit. its not more faster than 32 ?

The 64 bit builds are not optimized, and in tests the 32 bit versions have proved to have the advantage over 64 bit versions. For example see http://www.softblogger.net/64-bit-web-browser-speed-test

I would stick with the 32 bit builds until Mozilla make official 64 bit builds for Windows.