Mozilla ceased the development of Firefox 64-bit for Windows because many third party plug-ins were not having 64-bit versions.Mozilla Engineering Manager Benjamin Smedberg has now announced that he reviewed the negative feedback, consulted with his release engineering team, and has decided on a modification to the original plan. Firefox 64-bit for Windows may still never be released, but nightly builds will live another day.
The main reason for the change of plans appears to be that certain users regularly run into the 4GB memory limits of 32-bit builds due to hundreds or even thousands of tabs.
Source: Google Group