Firefox 64-bit builds for Windows have been available for testing as Developer Edition and Beta for a while, and starting with Firefox 42, the 64-bit builds are available to the public via the Release (stable) channel. This means a lot more users will start using the 64-bit version, and if your content requires a 32-bit NPAPI plug-in, it won’t work for those Firefox users. Currently Adobe Flash Player is the only supported plug-in on the 64-bit builds. Content publishers are strongly encouraged to move away from plug-in dependencies in favour of the latest Web technologies.
Update: The public release of Windows 64-bit builds has been postponed while Mozilla is “waiting for some partner changes”. The current timeline is unknown.
Update: Mozilla has decided to ship the 64-bit version with Firefox 43, enabling the Silverlight support as well.
Update: There is a bug on 64-bit builds where the file picker cannot be opened from Flash content. Mozilla is working with Adobe to solve the issue.