A firefox security update has been released for CentOS 6.

CentOS-announce: CESA-2020:1339 Critical CentOS 6 firefox Security Update

CentOS Errata and Security Advisory 2020:1339 Critical

Upstream details at :   https://access.redhat.com/errata/RHSA-2020:1339

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

i386:
459d53b5c53d356ac22b15f40bdb6d7c046d1ceb8bfda60a717325677780533f firefox-68.6.1-1.el6.centos.i686.rpm

x86_64:
459d53b5c53d356ac22b15f40bdb6d7c046d1ceb8bfda60a717325677780533f firefox-68.6.1-1.el6.centos.i686.rpm
e83b453b7ccedac2c51fe1c6885e7d93d94940bf9011bbba921b98e75b6d4bc7 firefox-68.6.1-1.el6.centos.x86_64.rpm

Source:
cb52c089eb51c556b126bca5243dfa28125152405f1e77de20dbc247c0288733 firefox-68.6.1-1.el6.centos.src.rpm