The February 2012 update to the Java Development Kit (JDK) and Java Runtime Environment (JRE) included a patch to correct a critical vulnerability that can permit the loading of arbitrary code on an end-users computer. To mitigate this risk, we have added affected versions of the Java plugin for Windows (Version 6 Update 30 and below as well as Version 7 Update 2 and below) to Firefox’s blocklist. The vulnerability is actively being exploited, and is a potential risk to users.”]
Source: https://blog.mozilla.org/addons/2012/04/02/blocking-java/