1) Download SUN's JRE from http://www.java.com/de/download/. Current version is 1.6.0_21.
2) Install it. I've installed it under /usr/local/java/jre1.6.0_21
3) Create the following symbolic link in /usr/lib/browser-plugins/ (root rights are needed)
sudo ln -s /usr/local/java/jre1.6.0_21/lib/i386/libnpjp2.so /usr/lib/browser-plugins/
4) Start Firefox and go to the Extras --> Add-ons --> Plugins
5) Deactivate IcedTea NPR Web Browser Plugin
Activate (if not yet activated) Java(TM) Plug-in 126.96.36.199