Add-on updates failed to verify that the add-on ID inside the signed package matched the ID of the add-on being updated. An attacker who could perform a man-in-the-middle attack on the user's connection to the update server and defeat the certificate pinning protection could provide a malicious signed add-on instead of a valid update. This vulnerability affects Firefox ESR < 45.5 and Firefox < 50.
http://rhn.redhat.com/errata/RHSA-2016-2780.html
https://bugzilla.mozilla.org/show_bug.cgi?id=1303418
https://security.gentoo.org/glsa/201701-15
https://www.mozilla.org/security/advisories/mfsa2016-89/