Since I always forget how to do this I figured I'd blog it for my own purposes so I don't have to sift through Google every time.
This assumes you have the JDK installed under /opt/java/jdk1.6.0_27 -- adjust accordingly if you have things installed elsewhere or only have the JRE.
Open up a terminal and do this:
sudo ln -s /opt/java/jdk1.6.0_27/jre/lib/amd64/libnpjp2.so
Then restart Firefox if it's running.
Now to look into doing this for Chrome ...