A firefox security update has been released for CentOS 6.

CentOS-announce: CESA-2020:3233 Important CentOS 6 firefox Security Update

CentOS Errata and Security Advisory 2020:3233 Important

Upstream details at :   https://access.redhat.com/errata/RHSA-2020:3233

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

i386:
7044f145b77b097e9563a8f3db28972097d7f4690d7a80199e49b49d3d61cc5e firefox-68.11.0-1.el6.centos.i686.rpm

x86_64:
7044f145b77b097e9563a8f3db28972097d7f4690d7a80199e49b49d3d61cc5e firefox-68.11.0-1.el6.centos.i686.rpm
8ee0612e553f9f3f549f841001f353e36807f2e803fc80e4e053e445d9eb741c firefox-68.11.0-1.el6.centos.x86_64.rpm

Source:
ef35f3d262b5627d8b0868c42ab00cb41ffd5709a07db75b5a16ab638020a3e7 firefox-68.11.0-1.el6.centos.src.rpm