64-bit version "Firefox" and "Thunderbird"

It is "Firefox" and "Thunderbird" which remodeled the code for the 64-bit version.

Mozilla for Windows x64 platform: Download
http://www.mozilla-x86-64.com/download.html
Most extensions seem to work, but the plug-in system is bad.

Here is a build method for running on Windows XP 64-bit version.

Mozilla for Windows x64 platform: How to Build Mozilla for Win 64
http://www.mozilla-x86-64.com/how_to_build.html