The version requiring no installation of 64-bit Firefox has now been released! The official name is Firefox (Minefield) Community Edition Release.
The downloaded file is a self-extracting archive, which, when executed, creates a folder named Firefox Community Edition at the selected location. This folder includes the file named firefox.exe, which is used to execute the 64-bit program. Thereafter, the software can be used as usual.
Firefox - 64bit software
Latest version: Firefox (Minefield) Community Edition 301 Win64 K8Q/64-X06 (LUT precache)
Release date: 2008-07-19
Download (Old version, not recommended)