My internet connection is behind a Cisco Firepower Threat Defense IPS which among other things performs SSL inspection via man in the middle attacks. The CA certificate for the FTD system has been added to both the operating system and Firefox's certificate store . Yet on one out of every 20 or 30 domains I visit I receive SSL_ERROR_DECRYPT_ERROR_ALERT errors when using HTTPS. If I refresh the page after receiving the error everything works fine. In addition I dont have this problem with other browsers such as Safari. Does anyone know why Firefox 54.0.1 seems to have intermittent trouble when SSL inspection is performed?