A possible fix for this problem: change the plugin that firefox uses.
After installing the sun jre1.6.0_11 package into /opt, the plugin that we stick into firefox is usually /opt/jre1.6.0_11/plugin/i386/ns7/libjavaplugin_oji.so
Well, use a different plugin instead: /opt/jre1.6.0_11/lib/i386/libnpjp2.so
This is the plugin that ubuntu sticks into firefox...and it works. So this is what I did to get it working. Open a terminal, become root, get rid of the old plugin, link to the new plugin:
# rm /usr/lib/mozilla/plugins/libjavaplugin_oji.so
# rm /home/myusername/.mozilla/plugins/libjavaplugin_oji.so
# ln -s /opt/jre1.6.0_11/lib/i386/libnpjp2.so /home/myusername/.mozilla/plugins/libnpjp2.so
Then I deleted my old /home/myusername/.java folder
Restarted firefox, went to the java chat site I was having problems with, and was able to log in. Maybe this will work for you too.