An existing mitigation of timing side-channel attacks is insufficient in some circumstances. This issue is addressed in Network Security Services (NSS) 3.26.1. This vulnerability affects Thunderbird < 45.5, Firefox ESR < 45.5, and Firefox < 50.
https://www.mozilla.org/security/advisories/mfsa2016-93/
https://www.mozilla.org/security/advisories/mfsa2016-90/
https://www.mozilla.org/security/advisories/mfsa2016-89/
https://www.debian.org/security/2016/dsa-3730
https://security.gentoo.org/glsa/201701-46
https://security.gentoo.org/glsa/201701-15
https://bugzilla.mozilla.org/show_bug.cgi?id=1293334