A thunderbird security update has been released for CentOS 6.

CentOS-announce: CESA-2020:4947 Important CentOS 6 thunderbird Security Update

CentOS Errata and Security Advisory 2020:4947 Important

Upstream details at :   https://access.redhat.com/errata/RHSA-2020:4947

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

i386:
8ec8ac5181b19be92bcc283c2950300f1a2f32afe97ad1da47a757d8febea355 thunderbird-78.4.0-1.el6.centos.i686.rpm

x86_64:
f477dfe604679c302feae6ee4ebb578e4329ffec6b8d1d63ba701700b4474c2f thunderbird-78.4.0-1.el6.centos.x86_64.rpm

Source:
2550e8fa8ec0b4787bd8dcfaa28d9a36c90e7f60f417cf0260a9d12dbbf3f09f thunderbird-78.4.0-1.el6.centos.src.rpm