Because the lazy bastards at Sun still didn't manage to make a 64 bit version of their Java plugin, you have to go through all sorts of hoops to make it actually work. Normally I wouldn't really care about this, but unfortunately my bank decided to require Java working in the browser for authentication. Four hours of my time later, I managed to get it working. To save others from some of the pain, here is how I did that:
I downloaded it to ~/install, so go into that directory and run:
chmod +x j2sdk-1.4.2-03-linux-amd64.bin
and wait until it's done installing (make sure it mentions "Uncompressing Blackdown Java 2 Standard Edition SDK v1.4.2-03" at some point).
Now, to make it work as a plugin, you have to link (not copy, as that makes the browser crash) the plugin to your mozilla directory. For me:
cd /home/derick/.mozilla/plugins ln -s /home/derick/install/j2sdk1.4.2⇢ /jre/plugin/amd64/mozilla/libjavaplugin_oji.so .
Restart the browser and check whether the blackdown java plugin shows up if you go to about:plugins .