On pages containing an iframe, the "data:" protocol can be used to create a modal alert that will render over arbitrary domains following page navigation, spoofing of the origin of the modal alert from the iframe content. This vulnerability affects Thunderbird < 52.3, Firefox ESR < 52.3, and Firefox < 55.
https://www.mozilla.org/security/advisories/mfsa2017-20/
https://www.mozilla.org/security/advisories/mfsa2017-19/
https://www.mozilla.org/security/advisories/mfsa2017-18/
https://www.debian.org/security/2017/dsa-3968
https://www.debian.org/security/2017/dsa-3928
https://security.gentoo.org/glsa/201803-14
https://bugzilla.mozilla.org/show_bug.cgi?id=1365875
https://access.redhat.com/errata/RHSA-2017:2534
https://access.redhat.com/errata/RHSA-2017:2456