Comment 6 for bug 288642

"GNU Class path's security implementation is not complete." is not a message from sun-java6-plugin. My guess would be the gcjwebplugin package, which uses either cacao or jamvm for the java runtime.

It is easy (and generally good that it's possible) to have multiple java runtimes installed at the same time. My understanding is that opera calls java directly without using a plugin, and it's probably finding the sun-java6-jre runtime even though the plugin in firefox is using the cacao (probably?) runtime.

Does this make a difference for you?

$ sudo aptitude install sun-java6-plugin
$ sudo update-java-alternatives -s java-6-sun