As stated on the website and in many mails on the list, CentOS stays in step with Redhat Enterprise release. Also on pages linked from the website, are places that have enhancements to the base release, later packages in some, added features in some others, ...
Take a look at the web site and FAQ and other things.
CentosPlus has a newer firefox than the base repository due to community demand, however it has become a bit outdated recently and does need to be rebuilt.