Starting with version 52 (March 2017) Firefox stopped supporting Silverlight
To use EPPI-Reviewer in Firefox (both Mac and Windows), you can use the Firefox ESR distribution
Firefox ESR is a release pathway optimised for stability and is therefore preferred in many business environments. Naturally, security isn't compromised, while stability is enhanced.
We are in the process of writing the next version of EPPI-Reviewer, which will run in a normal browser (without needing any plug-in).
In the mean time, EPPI-Reviewer can still be used on Macs (Safari and Firefox ESR) and Windows (Internet Explorer and Firefox ESR)
More information is here