Who is Remi?
I'm, for a few years, a fedora project contributor, indeed, I maintain various RPM for Fedora and EPEL repository and also some for RPMFusion
An exhaustive list on Fedora Package Database
I'm also a member of the PHP and EPEL SIGs (Special Interest Groups).
You can visit his site at http://blog.famillecollet.com/
rpm -ivh remi-release-6*.rpm epel-release-6*.rpm
yum --enablerepo=remi update thunderbird firefox
The remi reopository is not enabled by default, if you wish to change this edit the remi.repo file in your /etc/yum.repos.d folder.