If you have installed Firefox 3.6, 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