Mozilla Firefox before 49.0, Firefox ESR 45.x before 45.4, and Thunderbird < 45.4 rely on unintended expiration dates for Preloaded Public Key Pinning, which allows man-in-the-middle attackers to spoof add-on updates by leveraging possession of an X.509 server certificate for addons.mozilla.org signed by an arbitrary built-in Certification Authority.
Exploitability
AV:NAC:LPR:NUI:RScope
S:CImpact
C:HI:NA:N7.4/CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:N/A:N