WebExtensions bundled with embedded experiments were not correctly checked for proper authorization. This allowed a malicious WebExtension to gain full browser permissions. This vulnerability affects Firefox ESR < 60.1 and Firefox < 61.
https://www.mozilla.org/security/advisories/mfsa2018-16/
https://www.mozilla.org/security/advisories/mfsa2018-15/
https://usn.ubuntu.com/3705-1/
https://security.gentoo.org/glsa/201810-01
https://bugzilla.mozilla.org/show_bug.cgi?id=1454909