After I installed the plugin icedtea-gcjwebplugin_1.0-0ubuntu3 and the required packages, I made a symlink in /usr/lib/firefox-addons/plugins
to ln -s /usr/lib/jvm/java-6-openjdk/jre/lib/amd64/gcjwebplugin.so

My system is a 8.04 beta, after an online upgrade from 7.10 64bit.

The commands you asked give the following output:


stratos@homepc:~$ ls -l /usr/lib/xulrunner-addons/plugins/
σύνολο 44
lrwxrwxrwx 1 root root    46 2008-03-21 21:16 flashplugin-alternative.so -> /etc/alternatives/xulrunner-addons-flashplugin
lrwxrwxrwx 1 root root    45 2008-03-23 12:22 libjavaplugin.so -> /etc/alternatives/xulrunner-1.9-javaplugin.so
-rw-r--r-- 1 root root 21120 2008-03-13 20:34 libnullplugin.so
-rw-r--r-- 1 root root  5400 2008-03-22 23:57 librhythmbox-itms-detection-plugin.so
-rw-r--r-- 1 root root 10936 2008-03-13 20:34 libunixprintplugin.so
lrwxrwxrwx 1 root root    43 2008-03-21 21:07 mplayerplug-in-dvx.so -> ../../mozilla/plugins/mplayerplug-in-dvx.so
lrwxrwxrwx 1 root root    44 2008-03-21 21:07 mplayerplug-in-dvx.xpt -> ../../mozilla/plugins/mplayerplug-in-dvx.xpt
lrwxrwxrwx 1 root root    42 2008-03-21 21:07 mplayerplug-in-qt.so -> ../../mozilla/plugins/mplayerplug-in-qt.so
lrwxrwxrwx 1 root root    43 2008-03-21 21:07 mplayerplug-in-qt.xpt -> ../../mozilla/plugins/mplayerplug-in-qt.xpt
lrwxrwxrwx 1 root root    42 2008-03-21 21:07 mplayerplug-in-rm.so -> ../../mozilla/plugins/mplayerplug-in-rm.so
lrwxrwxrwx 1 root root    43 2008-03-21 21:07 mplayerplug-in-rm.xpt -> ../../mozilla/plugins/mplayerplug-in-rm.xpt
lrwxrwxrwx 1 root root    39 2008-03-21 21:07 mplayerplug-in.so -> ../../mozilla/plugins/mplayerplug-in.so
lrwxrwxrwx 1 root root    43 2008-03-21 21:07 mplayerplug-in-wmp.so -> ../../mozilla/plugins/mplayerplug-in-wmp.so
lrwxrwxrwx 1 root root    44 2008-03-21 21:07 mplayerplug-in-wmp.xpt -> ../../mozilla/plugins/mplayerplug-in-wmp.xpt
lrwxrwxrwx 1 root root    40 2008-03-21 21:07 mplayerplug-in.xpt -> ../../mozilla/plugins/mplayerplug-in.xpt

stratos@homepc:~$ ls -l /etc/alternatives/xulrunner-1.9-javaplugin.so
lrwxrwxrwx 1 root root 57 2008-03-23 12:22 /etc/alternatives/xulrunner-1.9-javaplugin.so -> /usr/lib/jvm/java-6-openjdk/jre/lib/amd64/gcjwebplugin.so

stratos@homepc:~$ update-alternatives --list xulrunner-1.9-javaplugin.so
/usr/lib/jvm/java-6-openjdk/jre/lib/amd64/gcjwebplugin.so

stratos@homepc:~$ sudo update-alternatives --config xulrunner-1.9-javaplugin.so
There is only 1 program which provides xulrunner-1.9-javaplugin.so
(/usr/lib/jvm/java-6-openjdk/jre/lib/amd64/gcjwebplugin.so). Nothing to configure.

stratos@homepc:~$ ls -l /etc/alternatives/xulrunner-1.9-javaplugin.so
lrwxrwxrwx 1 root root 57 2008-03-23 15:03 /etc/alternatives/xulrunner-1.9-javaplugin.so -> /usr/lib/jvm/java-6-openjdk/jre/lib/amd64/gcjwebplugin.so

stratos@homepc:~$ ls -l /usr/lib/xulrunner-addons/plugins/
σύνολο 44
lrwxrwxrwx 1 root root    46 2008-03-21 21:16 flashplugin-alternative.so -> /etc/alternatives/xulrunner-addons-flashplugin
lrwxrwxrwx 1 root root    45 2008-03-23 12:22 libjavaplugin.so -> /etc/alternatives/xulrunner-1.9-javaplugin.so
-rw-r--r-- 1 root root 21120 2008-03-13 20:34 libnullplugin.so
-rw-r--r-- 1 root root  5400 2008-03-22 23:57 librhythmbox-itms-detection-plugin.so
-rw-r--r-- 1 root root 10936 2008-03-13 20:34 libunixprintplugin.so
lrwxrwxrwx 1 root root    43 2008-03-21 21:07 mplayerplug-in-dvx.so -> ../../mozilla/plugins/mplayerplug-in-dvx.so
lrwxrwxrwx 1 root root    44 2008-03-21 21:07 mplayerplug-in-dvx.xpt -> ../../mozilla/plugins/mplayerplug-in-dvx.xpt
lrwxrwxrwx 1 root root    42 2008-03-21 21:07 mplayerplug-in-qt.so -> ../../mozilla/plugins/mplayerplug-in-qt.so
lrwxrwxrwx 1 root root    43 2008-03-21 21:07 mplayerplug-in-qt.xpt -> ../../mozilla/plugins/mplayerplug-in-qt.xpt
lrwxrwxrwx 1 root root    42 2008-03-21 21:07 mplayerplug-in-rm.so -> ../../mozilla/plugins/mplayerplug-in-rm.so
lrwxrwxrwx 1 root root    43 2008-03-21 21:07 mplayerplug-in-rm.xpt -> ../../mozilla/plugins/mplayerplug-in-rm.xpt
lrwxrwxrwx 1 root root    39 2008-03-21 21:07 mplayerplug-in.so -> ../../mozilla/plugins/mplayerplug-in.so
lrwxrwxrwx 1 root root    43 2008-03-21 21:07 mplayerplug-in-wmp.so -> ../../mozilla/plugins/mplayerplug-in-wmp.so
lrwxrwxrwx 1 root root    44 2008-03-21 21:07 mplayerplug-in-wmp.xpt -> ../../mozilla/plugins/mplayerplug-in-wmp.xpt
lrwxrwxrwx 1 root root    40 2008-03-21 21:07 mplayerplug-in.xpt -> ../../mozilla/plugins/mplayerplug-in.xpt
stratos@homepc:~$