An intermediate certificate was loaded into a man-in-the-middle (MITM) traffic management device. It was then used, during the process of inspecting traffic, to generate certificates for domains the device owner does not legitimately own or control. An attacker armed with a fraudulent SSL certificate and an ability to control their victims network could impersonate websites in a way that would be undetectable to most users. This change will be released to all supported versions of Firefox in the updates this week.”]
Source: https://blog.mozilla.org/security/2013/12/09/revoking-trust-in-one-anssi-certificate/