To install the Java plugin for Firefox, you have to create a symlink from Firefox's plugin directory /usr/lib/firefox-addons/plugins/ to the file libnpjp2.so in the JRE's “lib” dir.
sudo ln -s \ ${JAVA_HOME}/jre/lib/amd64/libnpjp2.so \ /usr/lib/firefox-addons/plugins/libnpjp2.so
Please note, that I tested this with Ubuntu 14.04, the “plugin” directory may be somewhere else on your system.