diff --git a/toolkit/components/extensions/ExtensionContent.jsm b/toolkit/components/extensions/ExtensionContent.jsm
index 026b099c84a5..d16ce6166dc3 100644
--- a/toolkit/components/extensions/ExtensionContent.jsm
+++ b/toolkit/components/extensions/ExtensionContent.jsm
@@ -215,11 +215,11 @@ Script.prototype = {
    */
   tryInject(window, sandbox, shouldRun, when) {
     if (shouldRun("document_start")) {
-      let winUtils = window.QueryInterface(Ci.nsIInterfaceRequestor)
-                           .getInterface(Ci.nsIDOMWindowUtils);
-
       let {cssURLs} = this;
       if (cssURLs.length > 0) {
+        let winUtils = window.QueryInterface(Ci.nsIInterfaceRequestor)
+                             .getInterface(Ci.nsIDOMWindowUtils);
+
         let method = this.remove_css ? winUtils.removeSheetUsingURIString : winUtils.loadSheetUsingURIString;
         for (let url of cssURLs) {
           runSafeSyncWithoutClone(method, url, winUtils.AUTHOR_SHEET);
diff --git a/toolkit/components/extensions/ExtensionManagement.jsm b/toolkit/components/extensions/ExtensionManagement.jsm
index dca162f4e355..324c5b71b933 100644
--- a/toolkit/components/extensions/ExtensionManagement.jsm
+++ b/toolkit/components/extensions/ExtensionManagement.jsm
@@ -239,8 +239,7 @@ var Service = {
 // extensionURIToAddonID, which ensures that we don't inject our
 // API into webAccessibleResources or remote web pages.
 function getAddonIdForWindow(window) {
-  let principal = window.document.nodePrincipal;
-  return principal.originAttributes.addonId;
+  return Cu.getObjectPrincipal(window).originAttributes.addonId;
 }
 
 const API_LEVELS = Object.freeze({