A thunderbird security update has been released for CentOS 6.

CentOS-announce: CESA-2020:3345 Important CentOS 6 thunderbird Security Update

CentOS Errata and Security Advisory 2020:3345 Important

Upstream details at :   https://access.redhat.com/errata/RHSA-2020:3345

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

i386:
5ca8ca5aed2e182a74276462102ae266b546b12bc4823ebcd7cc07b4412f741a thunderbird-68.11.0-1.el6.centos.i686.rpm

x86_64:
d00deca958deb6d40c61b780599f9d56822cca073952ad0db29041d2249d7c5e thunderbird-68.11.0-1.el6.centos.x86_64.rpm

Source:
3c8d27d6bbb365d6be581f8bddf126d8d3f6df8bfc173377a4189e5cb93c372d thunderbird-68.11.0-1.el6.centos.src.rpm