Firefox 17, coming out today on Beta, introduces a new solution for dealing with vulnerable or outdated plugins. Plugins that are blocked with the click-to-play flag will not load by default, but can be easily activated by users. This gives us a more user-friendly way to warn about plugins that should be updated, that give users better control over their browsing experience.
The large scale plugin update notification we deployed last week used the old update notification mechanism for Firefox 16 and below, and the new click-to-play mechanism for Firefox 17 and above. If you have old versions of Flash, Adobe Reader or Silverlight and you’re on 17, you will see the click-to-play block next time you visit a page that uses one of these plugins.
For more information see this post in the Security Blog.