Firefox gives the error "Your connection is not secure" when navigating to HTTPS sites through the Symantec Web Security Service (WSS). These sites work when using other browsers with the service such as Google, Chrome, or Microsoft Edge.
"Your connection is not secure"
Firefox uses it's own certificate store. This means that even if the SSL Root certificate has been installed to the Trusted Root Certification Authorities, Firefox won't use it.
Refer to Install SSL Root Certificate for Mozilla Firefox Browsers. The SSL Root Certificate must be imported from the Firefox certificate store.