First Install java runtime environment download it form Sun’s site
Then follow these steps
change to the plugins direcory of firefox
usually /usr/lib/firefox/plugins for firefox
/usr/lib/iceweasel/plugins for iceweasel
/usr/lib/mozilla/plugins for mozilla
/usr/lib/iceape/plugins for iceape
/usr/lib/epiphany/
Then execute the following commands as root or super user
$ln -s /opt/jre/plugin/i386/ns7/libjavaplugin_oji.so ./libjavaplugin_oji.so
I assume that you JRE directory is in /opt directory. Usually it is in /usr/lib/java or /usr/lib/jre etc…