Previously in Linux, enabling the java plugin was quite simple (if you knew where to find it), you simply put a link to the libjavaplugin_oji.so file into your ~/.mozilla/plugins directory.
Restart Firefox and you are good to go!
This is assuming you had Java installed. (I have the development kit installed in /opt/jdk directory). So this is what I used to do:
ln -s /opt/jdk/jre/lib/i386/libjavaplugin_oji.so
However, Firefox now requires a link to a different java plugin file - libnpjp2.so
You need to have java 1.6 update 10 or greater for this file to exist.
ln -s /opt/jdk/jre/lib/i386/libnpjp2.so
Once I added this link and restarted, Firefox now has Java Plugin working again.
By the Way...
I ended up keeping two versions of the JDK for this purpose.
I was wrong... the latest Versions of Java do have a 64 bit plugin!!! (I am currently running Java 1.6.0_18)
Use it with a 64-bit firefox build.
sudo ln -s /opt/jdk/jre/lib/amd64/libnpjp2.so
(or wherever your java is installed)
This also enables it in the 64-bit version of chromium!
I am running Ubuntu 9.1 amd64 version (on an Intel i7) so I get the chromium-daily 64-bit builds from http://ppa.launchpad.net/chromium-daily/ppa/ubuntu karmic main