Copied from http://www.mozilla-x86-64.com/:
Windows x64 (AMD64 or x86_64) build doesn't work
on 32bit Windows even if you use processor that
supports Intel EM64T or AMD64. If you want to use
this, you have to install Windows x64 version
(Windows XP Professional x64 Edition, Windows
Server 2003 x64 Edition or Windows Vista x64).
Firefox x64 version has to use 64-bit plug-in.
Even if you install 32-bit plug-in, it cannot use
it on 64-bit version.
Java??? Platform, Standard Edition 6u12 Releases
is ready for Windows x64 with plug-in support.
64-bit plugin is from 6u12 release.
Comments from the uploader (olofolleola4):
I wanted to upload the x64 unofficial edition of
Mozilla Firefox, because I don't think many people
knew that there is a x64 edition of Mozilla
The only Plugin right now that is working is the
latest Java Release.
That means for example that Shockwave Flash isn't
working, so you can't go toand other sites that
are using Shockwave Flash.
Test this x64 edition, and see if you like it ;) https://www.monova.org/torrent/2321683/Mozilla_Minefield_(Firefox)_3.0.6_x64_(64-bit).html