--- a/dom/fm/DOMFMRadioChild.js
+++ b/dom/fm/DOMFMRadioChild.js
@@ -41,23 +41,20 @@ DOMFMRadioChild.prototype = {
                flags: Ci.nsIClassInfo.DOM_OBJECT
              }),
 
   QueryInterface: XPCOMUtils.generateQI([Ci.nsIDOMFMRadio,
                                          Ci.nsIDOMGlobalPropertyInitializer]),
 
   // nsIDOMGlobalPropertyInitializer implementation
   init: function(aWindow) {
-    let principal = aWindow.document.nodePrincipal;
     let secMan = Cc["@mozilla.org/scriptsecuritymanager;1"]
                    .getService(Ci.nsIScriptSecurityManager);
 
-    let perm = (principal == secMan.getSystemPrincipal()) ?
-                 Ci.nsIPermissionManager.ALLOW_ACTION :
-                 Services.perms.testExactPermission(principal.URI, "fmradio");
+    let perm = Services.perms.testExactPermissionFromPrincipal(aWindow.document.nodePrincipal, "fmradio");
     this._hasPrivileges = perm == Ci.nsIPermissionManager.ALLOW_ACTION;
 
     if (!this._hasPrivileges) {
       throw new Components.Exception("Denied", Cr.NS_ERROR_FAILURE);
     }
 
     const messages = ["DOMFMRadio:enable:Return:OK",
                       "DOMFMRadio:enable:Return:NO",