Like a lot of people, I think, I installed Java on my year old Linux box without configuring FireFox to run it. I finally got round to it, its about 2 minutes work. Here’s how.
First, for running Java WebStart programs: select the *>System>Configuration>KDE>Components>File Associations menu. Then create a new group “Java WebStart” with the file association *.jnlp and the application /usr/java/jre/javaws/javaws (I keep the “jre” directory as a link to whatever the most recent version of the JRE is.) It should work pretty well straight away, and even download updated versions of the JRE (though it may put them in a strange place, at least if your /usr partition is getting down in size.)
Second, for running Java applets; go to the plugins/ directory of your firefox installation in a shell window, then link the plugin from the JRE to Firefox using the command
ln -s /usr/java/jre/plugin/i386/ns610-gcc32/libjavaplugin_oji.so
(again noting the “jre” may be different on your system.) If you are running the command in the KDE Konsole, a bad symbolic link shows up in flashing red. You will need to restart Firefox for it to take effect.
Oh, while you doing the File Associations, it is a good idea to make sure that *.jar is associated by /usr/java/jre/bin/java (whatever "jre" is on your system) so that executable jars will run too.