Here's what I saw when I visited the page on my Macbook:
A click on the "update" button and a restart of the browser patched my Silverlight Plug-In.
The Plugin Check service debuted in Firefox 3.5.3 with scans for vulnerable versions of the ubiquitous Flash Player plugin.
Separately, Mozilla has packaged a suite of security-centric plugins to help Firefox users stay safe while surfing on the Internet. The package, available here, includes NoScript, Better Privacy, AdBlock Plus and WOT (Web of Trust).