I never had this issue before, but here's what I did. tools > options > ttps > actions > reset with fiddler, and gave me "Added fiddler's root certificate to Machine Root List". On firefox, I did tools > options > Network settings > Configure Proxy Access to the Internet > user system proxy settings.
Another thing I tried is exporting the certificate from fiddler, then on firefox, tools > options > certificates > View certificates > import, then importing the fiddlerRoot.cer, but still get insecure connection.