On visiting site with a valid DigiCert SSL cert, FireFox 3.6.15 throws the following error Error code:Sec_error_unknown_issuer. The error was thrown on several different machines all running some version of Windows and running some version of FireFox - the issue did not get resolved by upgrading the FireFox to the latest version. Also note that IE or Chrome does not present any error while processing these sites. The issue was seen when SAML authentication was enabled for a hosted web proxy solution. The Error is thrown for both the hosted web proxy gateway as well as Internal SAML host. The sites are using Certs from DigiCert and Verisign respectively. Unable to replicate this issue outside of this environment.