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.
cd /usr/lib/mozilla/plugins
ls -l
Make sure that you do not have any libjavaplugin.so symlinks. If you do, you have to first unlink it:
unlink libjavaplugin.so
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.