rpm --import public.gpg.key

[root@ser ~]# yum update firefox

You have enabled checking of packages via GPG keys. This is a good thing.
However, you do not have any GPG public keys installed. You need to download
the keys for packages you wish to install and install them.
You can do that by running the command:
    rpm --import public.gpg.key
For more information contact your distribution or package provider.

解决方案:
rpm --import /usr/share/rhn/RPM-GPG-KEY