If you are trying to install from Firefox, you will need to install the Firefox Extended Support Release* version.
Normal editions of Firefox no longer support Silverlight.

* Firefox ESR version can be downloaded from the official site here.

* For technical details about Firefox Extended Support Release, please see this link.

Please Note:

You will need to visit this page from a compatible web browser for the Silverlight plugin to be enabled:

• Use Internet Explorer if installing from Windows.
• On an Apple system you will have to open this page in Firefox Extended Support Release*.
  Normal editions of Firefox no longer support Silverlight.

* Firefox ESR version can be downloaded from the official site here.

* For technical details about Firefox Extended Support Release, please see this link.