They are just hosting a old Nightly build from when 35.0a1 started on the Nightly development channel on September 2. There are four main development channels as in Release, Beta (b#), Aurora (a2) and Nightly (a1). The Aurora and Nightly channels gets checkins almost everyday so therefore they get a update each of those days. As a result the Nightly especially can sometimes be buggy due to chackin(s) that sometimes cause regressions and such until it is fixed or removed. Mozilla has yet to have Win64 for Release (Win32 is officially supported) so this site is just hosting random builds from Nightly channel when a new version starts on it. As early as next Monday or Tuesday there will be 36.0a1 starting on Nightly channel so that site will probably link to a 36.0a1 build then. If you want to be a Nightly Tester then do not get it from there (No guarantee its is a clean build and even if it is, you would get a large update to then be current) as get it from say [https://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/latest-mozilla-central/ here] instead if you want Win64. The supported Win32 is linked to on nightly.mozilla.org. Note that you should understand how to do a separate installs and using separate Profiles as you should not share the same Profile between a Release and Nightly. While Mozilla is looking to have [https://wiki.mozilla.org/Firefox/win64 Win64 for Release as early as Firefox 37.0 in first phase], these Win64 Nightlies are not officially supported in the same priority as Win32, Mac OSX 32and 64-bit combined and Linux 32-bit/64-bit builds get in maintenance of servers and builds.