I have always downloaded Java SDK from java.sun.com, but you can download also JRE only if you dont develop java code.
Then chmod +x to the j2...bin file and execute it it will self extract then move it to some location then make environment variables to the profile. i use /etc/profile.d/ dir and make there java.sh file that is executable and readable to all.
Content of java.sh:
Then make a symbolic link to the /usr/lib/mozilla/plugins folder like this:
ln -s /opt/j2sdk1.4.2_04/jre/plugin/i386/ns610-gcc32/libjavaplugin_oji.so /usr/lib/mozilla/plugins/libjavaplugin_oji.so
Where first is the plugin (target) second argument is the mozilla plugins folder.
Restart mozilla and it works
I use Fedora core test release....