At least for a while as Mozilla still plans on deprecating Silverlight support sometime in the future (much as Google and Microsoft already have). Bug 1225293 which was made public today indicates Mozilla’s short term plans to support Silverlight in the Win64 builds. When exactly Silverlight will return to the Firefox 64-Bit builds is not exactly known. It could (but not likely) be with Firefox 43 already in Beta and due out December 15th, 2015 or (more likely) Firefox 44 still in Developer’s Edition (Aurora) and due out in late January 2016 or Firefox 45 currently in Nightly builds and due out early March 2016.