Silverlight is no longer supported in Firefox version 52.0, Released on March 7, 2017.

Firefox no longer supports plugins that are based on NPAPI (Netscape Plugin Application Programming Interface) This includes Java, Adobe Acrobat, and Silverlight.

However, the "Extended Support Release" (ESR) builds of Firefox 52.0 still support NPAPI which means Silverlight will work with these builds.
You can find more information on  ESR here.
Firefox ESR can be downloaded from here.

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