su -c 'yum localinstall --nogpgcheck http://download1.rpmfusion.org/free/fedora/rpmfusion-free-release-18.noarch.rpm http://download1.rpmfusion.org/nonfree/fedora/rpmfusion-nonfree-release-18.noarch.rpm'
sudo yum install flash-plugin
This is by far the easiest way to
go about the installation. What those
two commands do is enabled a third-party
repository named rpmfusion, and then install
a package from there called flash-plugin that
will integrate with Firefox.