You have to point you MOZ_PLUGIN_PATH to where you keep your plugins, don't just copy it verbatim.
Lets back up, first can you even find the libjavaplugin_oji.so plugin? I unpacked java into the usr directory on my machine so here is where it is on my computer:
You need to find that file and make a symlink to it in your plugins directory.
It doesn't matter whats installed on you system, only what you use. I have 3 different version of Java on my machine, I just point to the different java binaries when I need a different version. Just curious, what is the output when you issue this command: