For some unknown reason the (K)Ubuntu developers did not update the Java plugin for firefox after jaunty (yet?).
The version that Karmic (9.10) pulls out of the multiverse repository is still jaunty's (9.04).
So when you try:
you'll get something like
Reading package lists... Done Building dependency tree Reading state information... Done Some packages could not be installed. This may mean that you have requested an impossible situation or if you are using the unstable distribution that some required packages have not yet been created or been moved out of Incoming. The following information may help to resolve the situation: The following packages have unmet dependencies: sun-java6-plugin: Depends: sun-java6-bin (= 6-15-1) but 6-16-0ubuntu1.9.04 is to be installed E: Broken packages
Actually if you have the Java Runtime Environment (JRE, package name
sun-java6-jre) installed all files needed are already present.
Just not put in the right place on the filesystem.
sudo ln -s /usr/lib/jvm/java-6-sun/jre/lib/amd64/libnpjp2.so /usr/lib/mozilla/plugins/
This will install the JRE (if it's not already installed) and will symlink the firefox plugin for java in place so that it'll be found after a browser restart.