The plans to ‘officially’ release a Win64 release of Firefox have now been pushed back to Firefox 42 (November 2015). As you may recall it was suppose to be Firefox 40 (August 2015) then was pushed to Firefox 41 (September 2015). The reason for the delay as described in Bug 1185532 has to do with the sandboxing feature for the 64-Bit Flash not working correctly.