Let's start with the download link to Firefox 4 rc1 for 64-bit Linux directory. You would think it would be as easy as going to the main download page for rc1. Nope only i686 Linux there. Next you might try a Google search with keywords like "firefox 4 rc1 64-bit". This gives you nothing but release notes, build help, and other miscellaneous links to the i686 version.
How did I find this ghost of build? I had to extrapolate the directory name from the i686 download directory. I figured if i686 was the 32-bit version then x86_64 had to be the 64-bit version. Really Mozilla did it have to be this hard?
Honestly this does not surprise me by now. Mozilla has not released a 64-bit build of Firefox until they started Firefox 4. I've been using the nightly builds since they started version 4. In the days before FF4 the only way to get a 64-bit build of Firefox was if you downloaded it from someone that built it (or you built it) or your Linux distro built one and you installed the package. This was better than nothing but you had to wait for security fixes from the distribution or the person doing the build.
FF4 is the first time Mozilla will be releasing an official 64-bit binary build of Firefox for Linux. I thank them for finally doing this. I just wish they would make 64-bit Firefox easier to find by putting it on the main downloads page.