You will need to copy the file
to /usr/lib/mozilla/plugins/ and then restart firefox.
I did the same on Ubuntu, installed java6 using synaptic, and it did not put a copy of the plugin where firefox can find it.
I used 'locate libnpjp2.so' to find the file in the first place. If you have never used locate, as root you have to run 'updatedb' first. That takes a few minutes to complete, so wait for it.
If you can not find that file, then you don't have java installed correctly.
You can download java from here-->http://www.java.com/en/download/linux_manual.jsp
and you can verify the java version, along with knowing if you have the latest version.