I had installed Java but it wasn't working in Firefox. In fact, I had tried both the binaries from Adobe and the binaries in Ubuntu's Synaptec. But, you might need to create a link to the plugin's .so file in Firefox's plugin directory. Here's how I did that.
First, update the locate database to include any files that have recently been installed.
sudo updatedb
Next, search for the Java plugin. I knew it would be called libjavaplugin or something similar, so I ran the following command.
locate libjavaplugin
And I got the following set of results
/opt/jre1.6.0_10/lib/i386/libjavaplugin_jni.so /opt/jre1.6.0_10/lib/i386/libjavaplugin_nscp.so /opt/jre1.6.0_10/lib/i386/libjavaplugin_nscp_gcc29.so /opt/jre1.6.0_10/plugin/i386/ns7/libjavaplugin_oji.so /opt/jre1.6.0_10/plugin/i386/ns7-gcc29/libjavaplugin_oji.so
I knew I was looking for the file with “oji” in it's name. There were 2, the ns7 and the ns7-gcc29 versions. I decided to used the former.
So, I switched to the directory I thought would contain Firefox and it did.
cd /usr/lib/firefox/plugins
Then I create a shortcut to the plugin.
sudo ln -s /opt/jre1.6.0_10/plugin/i386/ns7/libjavaplugin_oji.so