Bit of a puzzler
There are 32-bit and 64-bit versions of Nightly. But it’s the 32-bit one you should download. Then, you need to install it on a 64-bit version of Windows 8. That may sound odd, but it's true. For now, at least, Firefox on Modern is meant for 64-bit versions of Windows 8. To download Nightly 32-bit, look for the installer with "win32.installer.exe" in its filename.