Mozilla is changing the way Firefox loads third party plugins such as Flash, Java and Silverlight. This change will help increase Firefox performance and stability, and provide significant security benefits, while at the same time providing more control over plugins to our users.
Previously Firefox would automatically load any plugin requested by a website. Leveraging Click to Play Firefox will only load plugins when a user takes the action of clicking to make a particular plugin play or the user has previously configured Click To Play to always run plugins on the particular website.
read more.........https://blog.mozilla.org/security/2013/01/29/putting-users-in-control-of-plugins/