Linux howto: Enable Java runtime in Firefox

You need to
1) have jre installed
2) Edit > Preferences / [ Content ] tab: [ ] Enable Java checkbox checked
3)
$ cd /path/to/firefox/plugins
$ ln -s /path/to/java/jre/plugin/i386/ns7/libjavaplugin_oji.so ./
4) test

source

PS:
running .jnlp apps:
$ $JAVA_HOME/bin/javaws ./app.jnlp