For some reason the sun-java6-jre (or whatever Java you have installed) package in Hardy does not install it self as a plugin for Firefox-2
Perhaps there is a more elegant fix to this problem but I simply did
$ cd /usr/lib/firefox/plugins/
$ sudo ln -sf /etc/alternatives/iceweasel-javaplugin.so .
iceweasel-javaplugin can be controlled by update-alternatives