Firefox 52.0 has removed support for Silverlight.
For the Firefox release notes visit this link (see "changed" section). Why Firefox did this. The good news is there is a way to get around this which should work through early 2018. You must install the 64-bit version of Firefox ESR (Extended Support Release) in your locale. This worked as expected on my Windows 10 64-bit machine.
Firefox ESR x64 52.0 is the most currently supported version of Firefox that will work with Silverlight (at the time of this discussion post) .