http://www.engadget.com/2013/01/31/java-silverlight-disabled-by-default-firefox/
Java, Silverlight left in cold as Firefox disables all plugins by default -- except Flash
In an effort to prevent "drive-by exploitations," upcoming versions of Firefox will have Java, Adobe Reader and Silverlight disabled by default, according to a recent Mozilla Security blog. All other third-party plugins except Flash will also be disabled, requiring users to enable them using the so-called click to play feature introduced last year. All that is to prevent "poorly designed" Firefox plugins from crashing or recent headline-grabbing exploits involving the likes of Java, with Adobe's Flash player being the one exception that works out of the box -- though versions longer of tooth than 10.3 won't see daylight without your say-so.