Following a curiously short discussion, and despite 50% of Nightly testers on windows using the 64-bit builds[1] and a Windows user demonstrating firefox using 10GB of memory[2] (above the address space limit of 32-bit), the 64-bit nightly builds of Firefox on Windows are to be discontinued.

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1. https://bugzilla.mozilla.org/show_bug.cgi?id=814009

2. http://archive.installgentoo.net/g/thread/29261499

3. https://groups.google.com/forum/#!topic/mozilla.dev.apps.fir... - Full discussion thread