Sites can bypass security checks on permissions to install lightweight themes by manipulating the "baseURI" property of the theme element. This could allow a malicious site to install a theme without user interaction which could contain offensive or embarrassing images. This vulnerability affects Thunderbird < 52.8, Thunderbird ESR < 52.8, Firefox < 60, and Firefox ESR < 52.8.
https://www.debian.org/security/2018/dsa-4209
https://www.debian.org/security/2018/dsa-4199
https://usn.ubuntu.com/3660-1/
https://usn.ubuntu.com/3645-1/
https://security.gentoo.org/glsa/201811-13
https://security.gentoo.org/glsa/201810-01
https://lists.debian.org/debian-lts-announce/2018/05/msg00013.html
https://lists.debian.org/debian-lts-announce/2018/05/msg00007.html
https://access.redhat.com/errata/RHSA-2018:1726
https://access.redhat.com/errata/RHSA-2018:1725
https://access.redhat.com/errata/RHSA-2018:1415
https://access.redhat.com/errata/RHSA-2018:1414