ubuntu comes with icedtea / openjdk as the default java vm. this crewates some errors loading certain applets. the simple solution to fix that is to remove the icedtea plugin package and install the sun plugin package :
sudo apt-get remove icedtea6-plugin
sudo apt-get install sun-java6-plugin
now open firefox and open about:plugins to check that everything is ok.
in certain cases you may also need to do :
sudo update-alternatives --config java