A thunderbird security update has been released for CentOS 6.

CentOS-announce: CESA-2020:1488 Important CentOS 6 thunderbird Security Update

CentOS Errata and Security Advisory 2020:1488 Important

Upstream details at :   https://access.redhat.com/errata/RHSA-2020:1488

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

i386:
4627816b3ef77865943e788a494439844e17bd35463188c84a1cd5b4d66e6c03 thunderbird-68.7.0-1.el6.centos.i686.rpm

x86_64:
6e79ad52d3b218290c05103c8482ed06cebd6c8d96f792e51b3f6a2db4a7b9bf thunderbird-68.7.0-1.el6.centos.x86_64.rpm

Source:
ce31e6f5035cba3b118553bf78db9914fd2d62c6caf2e68d9a170be6048b8794 thunderbird-68.7.0-1.el6.centos.src.rpm