Java plugin should be enabled by default in Firefox when Fedora is installed. This is the way to check: open Firefox and type about:plugins in the address bar. Java plugin should be listed here. If not, its very easy to install it.
Make sure that you do not have any libjavaplugin.so symlinks. If you do, you have to first unlink it:
I am assuming you have a jre (either 1.6 or 1.7) already installed. You need to find the libnpjp2.so, and assuming its in /usr/lib/jdk1.6.0_14/jre/lib/i386/libnpjp2.so,
ln -s /usr/lib/jdk1.6.0_14/jre/lib/i386/libnpjp2.so
Restart Firefox and you should be all set.