Starting in Java 7, the Java Control Panel includes the Security option, "Enable Java content in the browser", which is selected by default. De-selecting this option will prevent Java applets from running in any browser and the Java plugin will no longer appear in the Firefox Add-ons Manager. Make sure that Enable Java content in the browser is selected in the Java Control Panel Security tab. See this Java Help page for details.Make sure the 32-bit version of Java is installed