A good source for plugins for all types of mozilla-type browsers
is http://plugindoc.mozdev.org/.
From there you follow the link to your particular operating system.
Personally, I had mozilla (version 1.7.8) running under Linux but without
java support.
I also had installed the complete Java Development Kit ( jdk1.5.0_10 ) for the
purpose of developing java applications. As a result it was not necessary
to obtain any additional files, I just had to enable mozilla appropriately.
Following the instructions given at http://plugindoc.mozdev.org/linux.html#Java I had to identify
the directory in which the JDK has the requested .so file :
/usr/java/jdk1.5.0_10/jre/plugin/i386/ns7/libjavaplugin_oji.so
and where mozilla has its plugins directory :
/usr/lib/mozilla-1.7.8/plugins
As root :
cd /usr/lib/mozilla-1.7.8/plugins
ln -s /usr/java/jdk1.5.0_10/jre/plugin/i386/ns7/libjavaplugin_oji.so libjavaplugin_oji.so