Google notified Mozilla that an intermediate certificate, which chains up to a root included in Mozilla’s root store, was loaded into a man-in-the-middle (MITM) traffic management device. This certificate was issued by Agence nationale de la sécurité des systèmes d'information (ANSSI), an agency of the French government and a certificate authority in Mozilla's root program. A subordinate certificate authority of ANSSI mis-issued an intermediate certificate that they installed on a network monitoring device, which enabled the device to act as a MITM proxy performing traffic management of domain names or IP addresses that the certificate holder did not own or control.
The issue was not specific to Firefox but there was evidence that one of the certificates was used for MITM traffic management of domain names that the customer did not legitimately own or control. This issue was resolved by revoking trust in the intermediate used by the sub-CA to issue the certificate for the MITM device.