Nope, they're just misinformed; they read the roadmap incorrectly. What the Firefox devs mean is a 64-bit version of Firefox (i.e. an x64 process). Right now Firefox runs perfectly on both Windows 7 and 64-bit versions of Windows.
There are 32-bit and 64-bit builds of Firefox. The 64-bit builds only work on 64-bit machines -- and the 32-bit builds work on both 32- and 64-bit machines.
The usual proviso apply of course: these are nightly builds and come with no guarentees, nor should they be used as an indication of the quality of the release going forwards.
Nope, they're just misinformed; they read the roadmap incorrectly. What the Firefox devs mean is a 64-bit version of Firefox (i.e. an x64 process). Right now Firefox runs perfectly on both Windows 7 and 64-bit versions of Windows.