The applet seems to work only on Sun's JRE. It was tested against jre 1.5.0_08 and it worked fine. It is known not to work on IcedTea and gcj java.
The applet may not start throwing an exception:
class nn.pp.rc.RemoteConsoleApplet.class not found. java.lang.ClassNotFoundException: nn.pp.rc.RemoteConsoleApplet.class
For some reason, the applet jars do not load properly. However, I managed to manually copy them to the jar cache directory, from another computer where everything works fine. You need to extract the files in the following archive to the path below:
With a standard firefox (2.x) installation applet code is cached in the following location:
~/.java/deployment/cache/javapi/v1.0/jar
The cache directory is created the first time a java applet is run.
There is an issue with some version of firefox and older version of xorg libraries, which result in not being able to run applets at all (although things look fine in the “about:plugins” page). Particularly, there is an issue with libxcb, which (at least at version 1.0) “does not allow applications that are not thread safe to run”. Upgrading to version 1.1 solved the issues for me.