https://rhn.redhat.com/errata/RHSA-2005-176.html refers:

This update has been rated as having critical security impact by the CentOS Security Response Team.

The following updated file has been uploaded and is currently syncing to the mirrors:

i386:
firefox-1.0.1-1.4.3.centos4.1.i386.rpm

IA64:
firefox-1.0.1-1.4.3.centos4.1.ia64.rpm

You may update your CentOS-4 installations by running the command:

yum upgrade