I am using Kubuntu 10.04 with Firefox 3.6 from Mozilla's website and I use OpenJDK from the Ubuntu repsitories.
I want to know how do I configure OpenJDK with the Firefox to view Java applets in the browser.
Just install a package called icedtea-6-plugin (or icedtea-7-plugin for OpenJDK 7) and you should be all set.
You can verify it works correctly on this page: http://www.java.com/en/download/testjava.jsp
If you're using Fedora like me you need to install
sudo yum install icedtea-web
Restart your browser after this.