I created a prerelase repository for Centos 5 

In order to validate that the packages came from me, you'll want to install my public GPG key on your system. You can take care of that with a single command as root:

rpm --import http://makowski.fedorapeople.org/fb21/centos/RPM-GPG-KEY-IBPhoenix

Then create the following file to enabled the repository :

/etc/yum.repos.d/pmakowski.repo

The content of this file should be  for x86_64 :

[pmakowski]

name=Philippe Makowski Repo

baseurl=http://makowski.fedorapeople.org/fb21/centos/5/x86_64/

enabled=1

gpgcheck=1

gpgkey=http://makowski.fedorapeople.org/fb21/centos/RPM-GPG-KEY-IBPhoenix

The content of this file should be  for i386 :

[pmakowski]

name=Philippe Makowski Repo

baseurl=http://makowski.fedorapeople.org/fb21/centos/5/i386/

enabled=1

gpgcheck=1

gpgkey=http://makowski.fedorapeople.org/fb21/centos/RPM-GPG-KEY-IBPhoenix

That's all, you can use yum or pirut to install or update Firebird 2.1