CentOS Errata and Security Advisory 2008:0598
https://rhn.redhat.com/errata/RHSA-2008-0598.html
The following updated files have been uploaded and are currently syncing to the mirrors:
s390: updates/s390/RPMS/firefox-1.5.0.12-0.21.el4.centos.s390.rpm
s390x: updates/s390x/RPMS/firefox-1.5.0.12-0.21.el4.centos.s390x.rpm