A thunderbird security update has been released for CentOS 6.

CentOS Errata and Security Advisory 2020:0914 Important

Upstream details at :   https://access.redhat.com/errata/RHSA-2020:0914

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

i386:
b0fbb6aef153f906cbe4af6a0603f9170815003c466a76f477d2ccb3e3c6db15 thunderbird-68.6.0-1.el6.centos.i686.rpm

x86_64:
565ec9f4b301d92ffebce309e90fefa2b5fd73ef43f7589aa54dbc9268c4ea59 thunderbird-68.6.0-1.el6.centos.x86_64.rpm

Source:
1d34e6136fe8c88528670dbef5e90bbfd790b0b8b695dc18d5cb6b19edc7e5d5 thunderbird-68.6.0-1.el6.centos.src.rpm