If installed systemwide, Java installs to /usr/java/j2re1.4.2_06/ (for j2re 1.4.2_06)
This installation must be completed as the root user, or java will intall into your $USER directory. The java plugins are in /usr/java/j2re1.4.2_06/plugin/i386/ns610-gcc32 (or ns610, or ns4).
Firefox looks for the java plugin in /usr/lib/firefox/plugins
To make the java plugins available to Firefox, link the plugin file to the plugin directory under firefox.