// lookup xpcom components
var certDB = Components.classes["@mozilla.org/security/x509certdb;1"].
                getService(Components.interfaces.nsIX509CertDB); 
var aFile = Components.classes["@mozilla.org/file/local;1"].
                createInstance(Components.interfaces.nsILocalFile];

var tokenDB = Components.classes["@mozilla.org/security/pk11tokendb;1"].
                getService(Components.interfaces.nsIPK11TokenDB);

// make sure that the certificate store will not require a master password
if(tokenDB.getInternalKeyToken().needsUserInit)
{
   tokenDB.getInternalKeyToken().initWithPassword(null);
}

// initialize the file to the path for the cert
aFile.initWithPath(certFileName);

// load the cert
certDB.importPKCS12File(null, aFile);