Over the past two weeks, Trevor, Alex and I worked on 64-bit support for Firefox on Windows. I am pleased to announce that we were successful, and that Win-64 versions of Firefox Nightly builds should now work with screen readers. So if you have a 64-bit edition of Windows 7, 8.x or 10 Preview, and run NVDA, JAWS, Window-Eyes or other screen readers that support Firefox, you should be able to uninstall the 32-bit version of Firefox Nightly if you have it installed, and download and run the 64-bit installer from the above linked page.
It is expected that everything works as in the 32-bit version. If, for some reason, you find oddities, we do want to know about them ASAP. We plan to backport support to Firefox Dev Edition (currently at version 40), and maybe 39 beta, but the latter is not certain yet.
So if you use Nightly anyway, and would like to help, we definitely appreciate you switching over to the 64 bit edition and give us feedback in the comments below! Thanks!