--- a/toolkit/components/extensions/ExtensionContent.jsm
+++ b/toolkit/components/extensions/ExtensionContent.jsm
@@ -267,18 +267,16 @@ class ExtensionContext extends BaseConte
     let contentPrincipal = contentWindow.document.nodePrincipal;
     let ssm = Services.scriptSecurityManager;
 
     // copy origin attributes from the content window origin attributes to
     // preserve the user context id. overwrite the addonId.
     let attrs = contentPrincipal.originAttributes;
     attrs.addonId = this.extension.id;
     let extensionPrincipal = ssm.createCodebasePrincipal(this.extension.baseURI, attrs);
-    Object.defineProperty(this, "principal",
-                          {value: extensionPrincipal, enumerable: true, configurable: true});
 
     if (ssm.isSystemPrincipal(contentPrincipal)) {
       // Make sure we don't hand out the system principal by accident.
       // also make sure that the null principal has the right origin attributes
       prin = ssm.createNullPrincipal(attrs);
     } else {
       prin = [contentPrincipal, extensionPrincipal];
     }
@@ -317,16 +315,22 @@ class ExtensionContext extends BaseConte
 
       Cu.evalInSandbox(`
         window.JSON = JSON;
         window.XMLHttpRequest = XMLHttpRequest;
         window.fetch = fetch;
       `, this.sandbox);
     }
 
+    Object.defineProperty(this, "principal", {
+      value: Cu.getObjectPrincipal(this.sandbox),
+      enumerable: true,
+      configurable: true,
+    });
+
     let url = contentWindow.location.href;
     // The |sender| parameter is passed directly to the extension.
     let sender = {id: this.extension.uuid, frameId, url};
     let filter = {extensionId: this.extension.id};
     let optionalFilter = {frameId};
     this.messenger = new Messenger(this, [this.messageManager], sender, filter, optionalFilter);
 
     this.chromeObj = Cu.createObjectIn(this.sandbox, {defineAs: "browser"});