Mozilla has announced they will be blocking older versions of the Java Runtime Environment (JRE) and Java Development Kit (JDK) plugins due to critical vulnerabilities that are being exploited.
This vulnerability—present in the older versions of the JDK and JRE—is actively being exploited, and is a potential risk to users. 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. A blocklist entry for the Java plugin on OS X may be added at a future date.
User can to check to see if they need to update their JAVA (or any others such as Flash) plugins by going to the Mozilla’s plugin checker site.
Source: Mozilla Add-ons Blog