A thunderbird security update has been released for CentOS 6.

CentOS Errata and Security Advisory 2020:0123 Important

Upstream details at :   https://access.redhat.com/errata/RHSA-2020:0123

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

i386:
34b6df5def4bed3126d9eb1b86fddc7952cf5625a222bc67b059631641844db5 thunderbird-68.4.1-2.el6.centos.i686.rpm

x86_64:
d48c7fbbd5318de3282522bf6bd4721937bcc875d40d30deabca26aac033b949 thunderbird-68.4.1-2.el6.centos.x86_64.rpm

Source:
dccfa064d06d90cf67c2d7b499c3b6e47b04006abee44f69ed07f5f6767ad732 thunderbird-68.4.1-2.el6.centos.src.rpm