1) Test Java. Podemos ver si funciona java en:
http://www.java.com/es/download/help/testvm.xml

http://www.javatester.org/version.html

2) Firefox. Ver los plugins en:
about:plugins

3) Plugin Instalación:
http://plugindoc.mozdev.org/linux.html#Java

Java Runtime Environment
Version: 1.4.2 or later
SeaMonkey 1.0: Supported
Firefox 2.0: Supported
FAQ: Java FAQ

1. Install Java Runtime Environment.
2. Make a symbolic link to libjavaplugin_oji.so in your Mozilla Plugins directory. Use the copy located in the plugin/i386/ns7 directory of JRE 5.0 or later, or plugin/i386/ns610-gcc32 if you are using JRE 1.4.2.

Important! Do not copy the plugin to your plugins directory. If you do, Mozilla will crash any time you attempt to view a page containing a Java applet.

4)Instalar java:

Dercargando archivo: jre-6u19-linux-i586.bin

De la web: http://www.java.com/es/download/linux_manual.jsp?locale=es&host=www.java.com

Desde los repositorios:
sudo aptitude install jamvm openjdk-6-jre-headless sun-java6-jre sun-java6-plugin sun-java6-fonts

5) Arreglar enlaces:
ln -s /usr/lib/jvm/jre1.6.0_18/plugin/i386/ns7/libjavaplugin_oji.so $HOME/firefox/plugins/
sudo ln -s /usr/lib/jvm/jre1.6.0_18/plugin/i386/ns7/libjavaplugin_oji.so /usr/lib/firefox/plugins/

6) Ver plugins aplicados:
sudo galternatives