A firefox security update has been released for CentOS 6.

CentOS-announce: CESA-2020:4330 Important CentOS 6 firefox Security Update

CentOS Errata and Security Advisory 2020:4330 Important

Upstream details at :   https://access.redhat.com/errata/RHSA-2020:4330

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

i386:
a8e5176d70f304e213059440333fe1b5e3a057c081d7b6543f2da13d2887eabe firefox-78.4.0-2.el6.centos.i686.rpm

x86_64:
a8e5176d70f304e213059440333fe1b5e3a057c081d7b6543f2da13d2887eabe firefox-78.4.0-2.el6.centos.i686.rpm
4d097a2864cc6adebfd0005683826399f45a9c13edeba2b70e5469340401c4ec firefox-78.4.0-2.el6.centos.x86_64.rpm

Source:
5672b6694ef983b8fa988f02c618bf300a74fe64533a228f2d7252b736673a4d firefox-78.4.0-2.el6.centos.src.rpm