If you have installed Firefox 3.6 (for instance from the Ubuntu Mozilla Daily Build Team PPA) you will discover that Java is no longer working
For i386 (32 bit Ubuntu):
$ sudo update-alternatives --install /usr/lib/mozilla/plugins/libjavaplugin.so mozilla-javaplugin.so /usr/lib/jvm/java-6-sun/jre/lib/i386/libnpjp2.so 50
For amd64 (64 bit Ubuntu):
$ sudo update-alternatives --install /usr/lib/mozilla/plugins/libjavaplugin.so mozilla-javaplugin.so /usr/lib/jvm/java-6-sun/jre/lib/amd64/libnpjp2.so 50
