yum can only install rpms from the designated repos in the dir /etc/yum.repos.d.
On my Centos 6.3 box it has
rpm -qa|grep firefox
What I recommend is that you (using another computer) go to the Centos site and download/burn Centos 6.3 DVD isos, then use this https://code.google.com/p/iredmail/w...Yum_Repository
to create a local repo and 'yum update' from there.
Note that RH (& therefore Centos) backport a lot of fixes, so the major version number doesn't tell the whole story.
However, if you really NEED eg FF16, then you will have to go to the Firefox home site and install a download from there. Dump the file into eg /opt/ff16, then extract and follow the instructions therein.