A firefox security update has been released for CentOS 6.

CentOS-announce: CESA-2020:3558 Important CentOS 6 firefox Security Update

CentOS Errata and Security Advisory 2020:3558 Important

Upstream details at :   https://access.redhat.com/errata/RHSA-2020:3558

The following updated files have been uploaded and are currently
syncing to the mirrors: ( sha256sum Filename )

i386:
7aaf26cdf6b6fe4bbc989ca9afc2b9a52bbde92a1fc3718cd7ed90d5a5d0875b firefox-68.12.0-1.el6.centos.i686.rpm

x86_64:
7aaf26cdf6b6fe4bbc989ca9afc2b9a52bbde92a1fc3718cd7ed90d5a5d0875b firefox-68.12.0-1.el6.centos.i686.rpm
49b77ef47320110d89f10574d0cc64337dd3e1d704e360de6c09081e8727841b firefox-68.12.0-1.el6.centos.x86_64.rpm

Source:
2be4234b61c9ddbbee04f99bafb4118674334a1dd03cfd8fe2993eadbb594f49 firefox-68.12.0-1.el6.centos.src.rpm