A thunderbird security update has been released for CentOS 6.

CentOS Errata and Security Advisory 2020:0574 Important

Upstream details at :   https://access.redhat.com/errata/RHSA-2020:0574

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

i386:
0ecd700cd71a96565913e36ef8350995a5d00cccc830f3042f00fbebd833fdf2 thunderbird-68.5.0-1.el6.centos.i686.rpm

x86_64:
0f8583b47449d42711d8df5daeb3e9a2ec217c1304e771a711b8d775673f60f5 thunderbird-68.5.0-1.el6.centos.x86_64.rpm

Source:
bb00e7c71ec579f52dbb33f9fef9676a2b27b49250e451798f9a4c8ef765f927 thunderbird-68.5.0-1.el6.centos.src.rpm