An attacker could use a JavaScript Map/Set timing attack to determine whether an atom is used by another compartment/zone in specific contexts. This could be used to leak information, such as usernames embedded in JavaScript code, across websites. This vulnerability affects Firefox < 50.1, Firefox ESR < 45.6, and Thunderbird < 45.6.
https://www.mozilla.org/security/advisories/mfsa2016-96/
https://www.mozilla.org/security/advisories/mfsa2016-95/
https://www.mozilla.org/security/advisories/mfsa2016-94/
https://www.debian.org/security/2017/dsa-3757
https://security.gentoo.org/glsa/201701-15
https://bugzilla.mozilla.org/show_bug.cgi?id=1317936
http://www.securitytracker.com/id/1037461