Silverlight plugin 5.1.41105.0 and lower (click to play) has been blocked for your protection.

Why was it blocked?
Old versions of the Silverlight plugin are known to have critical security vulnerabilities. You can get the latest version of Silverlight here.
Who is affected?
All users who have these versions of the Silverlight plugin installed.
What does this mean?

The problematic add-on or plugin will be automatically disabled and no longer usable.

When Mozilla becomes aware of add-ons, plugins, or other third-party software that seriously compromises Firefox security, stability, or performance and meets certain criteria, the software may be blocked from general use. For more information, please read this support article.