A firefox security update has been released for CentOS 6.

CentOS-announce: CESA-2020:1429 Important CentOS 6 firefox Security Update

CentOS Errata and Security Advisory 2020:1429 Important

Upstream details at :   https://access.redhat.com/errata/RHSA-2020:1429

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

i386:
92fa23b14ba91a20f2d063b24670606180b0f9aa2e572c3e6546665a7a51e6e3 firefox-68.7.0-2.el6.centos.i686.rpm

x86_64:
92fa23b14ba91a20f2d063b24670606180b0f9aa2e572c3e6546665a7a51e6e3 firefox-68.7.0-2.el6.centos.i686.rpm
98360b8ff2dc8ec86abefe993884713c4bcf323d8ba353b9dd61e18d9d143e79 firefox-68.7.0-2.el6.centos.x86_64.rpm

Source:
f59d98c6aba1c8a29536305d03ebb0f82fa35128a959856616ccd142cfb9d763 firefox-68.7.0-2.el6.centos.src.rpm