diff --git a/content/base/src/nsGkAtomList.h b/content/base/src/nsGkAtomList.h
index 15724e0cf5e..3329842b82e 100644
--- a/content/base/src/nsGkAtomList.h
+++ b/content/base/src/nsGkAtomList.h
@@ -342,7 +342,6 @@ GK_ATOM(events, "events")
 GK_ATOM(excludeResultPrefixes, "exclude-result-prefixes")
 GK_ATOM(excludes, "excludes")
 GK_ATOM(expr, "expr")
-GK_ATOM(expectingSystemMessage, "expecting-system-message")
 GK_ATOM(extends, "extends")
 GK_ATOM(extensionElementPrefixes, "extension-element-prefixes")
 GK_ATOM(face, "face")
diff --git a/content/html/content/src/nsGenericHTMLFrameElement.cpp b/content/html/content/src/nsGenericHTMLFrameElement.cpp
index de0ab334ba5..30deb40f1bf 100644
--- a/content/html/content/src/nsGenericHTMLFrameElement.cpp
+++ b/content/html/content/src/nsGenericHTMLFrameElement.cpp
@@ -321,19 +321,6 @@ nsGenericHTMLFrameElement::GetReallyIsApp(bool *aOut)
   return NS_OK;
 }
 
-/* [infallible] */ NS_IMETHODIMP
-nsGenericHTMLFrameElement::GetIsExpectingSystemMessage(bool *aOut)
-{
-  *aOut = false;
-
-  if (!nsIMozBrowserFrame::GetReallyIsApp()) {
-    return NS_OK;
-  }
-
-  *aOut = HasAttr(kNameSpaceID_None, nsGkAtoms::expectingSystemMessage);
-  return NS_OK;
-}
-
 NS_IMETHODIMP
 nsGenericHTMLFrameElement::GetAppManifestURL(nsAString& aOut)
 {
diff --git a/dom/interfaces/html/nsIMozBrowserFrame.idl b/dom/interfaces/html/nsIMozBrowserFrame.idl
index fedbec09333..973202d5c49 100644
--- a/dom/interfaces/html/nsIMozBrowserFrame.idl
+++ b/dom/interfaces/html/nsIMozBrowserFrame.idl
@@ -30,16 +30,6 @@ interface nsIMozBrowserFrame : nsIDOMMozBrowserFrame
    */
   [infallible] readonly attribute boolean reallyIsApp;
 
-  /**
-   * This corresponds to the expecting-system-message attribute, which tells us
-   * whether we should expect that this frame will receive a system message once
-   * it starts up.
-   *
-   * It's the embedder's job to set this attribute on a frame.  Its presence
-   * might cause us to increase the priority of the frame's process.
-   */
-  [infallible] readonly attribute boolean isExpectingSystemMessage;
-
   /**
    * Gets this frame's app manifest URL, if the frame really is an app frame.
    * Otherwise, returns the empty string.