diff --git a/toolkit/content/browser-child.js b/toolkit/content/browser-child.js
index a5a79115db3f..9d7d9a7657e0 100644
--- a/toolkit/content/browser-child.js
+++ b/toolkit/content/browser-child.js
@@ -140,6 +140,7 @@ let WebProgressListener = {
       json.charset = content.document.characterSet;
       json.mayEnableCharacterEncodingMenu = docShell.mayEnableCharacterEncodingMenu;
       json.principal = content.document.nodePrincipal;
+      json.synthetic = content.document.mozSyntheticDocument;
     }
 
     sendAsyncMessage("Content:LocationChange", json, objects);
@@ -326,22 +327,6 @@ addEventListener("ImageContentLoaded", function (aEvent) {
   }
 }, false);
 
-let DocumentObserver = {
-  init: function() {
-    Services.obs.addObserver(this, "document-element-inserted", false);
-    addEventListener("unload", () => {
-      Services.obs.removeObserver(this, "document-element-inserted");
-    });
-  },
-
-  observe: function(aSubject, aTopic, aData) {
-    if (aSubject == content.document) {
-      sendAsyncMessage("DocumentInserted", {synthetic: aSubject.mozSyntheticDocument});
-    }
-  },
-};
-DocumentObserver.init();
-
 const ZoomManager = {
   get fullZoom() {
     return this._cache.fullZoom;
diff --git a/toolkit/content/tests/browser/browser.ini b/toolkit/content/tests/browser/browser.ini
index 438468297907..2cb88db11795 100644
--- a/toolkit/content/tests/browser/browser.ini
+++ b/toolkit/content/tests/browser/browser.ini
@@ -18,6 +18,9 @@ skip-if = e10s
 skip-if = e10s # Disabled for e10s: Bug ?????? - seems to be a timing issue with RemoteFinder.jsm messages coming later than the tests expect.
 [browser_input_file_tooltips.js]
 skip-if = e10s # Bug ?????? - test directly manipulates content (TypeError: doc.createElement is not a function)
+[browser_isSynthetic.js]
+support-files =
+  empty.png
 [browser_keyevents_during_autoscrolling.js]
 skip-if = e10s # Bug 921935 - focusmanager issues with e10s
 [browser_save_resend_postdata.js]
diff --git a/toolkit/content/tests/browser/browser_isSynthetic.js b/toolkit/content/tests/browser/browser_isSynthetic.js
new file mode 100644
index 000000000000..9bea87e798f1
--- /dev/null
+++ b/toolkit/content/tests/browser/browser_isSynthetic.js
@@ -0,0 +1,60 @@
+function waitForLoad(browser) {
+  return new Promise(resolve => {
+    let wasSynthetic = browser.isSyntheticDocument;
+
+    browser.addProgressListener({
+      onLocationChange: function(webProgress, request, location, flags) {
+        wasSynthetic = browser.isSyntheticDocument;
+      },
+
+      onStateChange: function(webProgress, request, flags, status) {
+        let docStop = Ci.nsIWebProgressListener.STATE_IS_NETWORK |
+                      Ci.nsIWebProgressListener.STATE_STOP;
+        if ((flags & docStop) == docStop) {
+          browser.removeProgressListener(this);
+          resolve(wasSynthetic);
+        }
+      },
+
+      QueryInterface: XPCOMUtils.generateQI([Ci.nsIWebProgressListener,
+                                             Ci.nsISupportsWeakReference])
+    });
+  });
+}
+
+const FILES = gTestPath.replace("browser_isSynthetic.js", "")
+                       .replace("chrome://mochitests/content/", "http://example.com/");
+
+add_task(function*() {
+  let tab = gBrowser.addTab("about:blank");
+  let browser = tab.linkedBrowser;
+  yield waitForLoad(browser);
+
+  is(browser.isSyntheticDocument, false, "Should not be synthetic");
+
+  let loadPromise = waitForLoad(browser);
+  browser.loadURI("data:text/html;charset=utf-8,<html/>");
+  let wasSynthetic = yield loadPromise;
+  is(wasSynthetic, false, "Should not be synthetic");
+  is(browser.isSyntheticDocument, false, "Should not be synthetic");
+
+  loadPromise = waitForLoad(browser);
+  browser.loadURI(FILES + "empty.png");
+  wasSynthetic = yield loadPromise;
+  is(wasSynthetic, true, "Should be synthetic");
+  is(browser.isSyntheticDocument, true, "Should be synthetic");
+
+  loadPromise = waitForLoad(browser);
+  browser.goBack();
+  wasSynthetic = yield loadPromise;
+  is(wasSynthetic, false, "Should not be synthetic");
+  is(browser.isSyntheticDocument, false, "Should not be synthetic");
+
+  loadPromise = waitForLoad(browser);
+  browser.goForward();
+  wasSynthetic = yield loadPromise;
+  is(wasSynthetic, true, "Should be synthetic");
+  is(browser.isSyntheticDocument, true, "Should be synthetic");
+
+  gBrowser.removeTab(tab);
+});
diff --git a/toolkit/content/tests/browser/empty.png b/toolkit/content/tests/browser/empty.png
new file mode 100644
index 000000000000..17ddf0c3ee9a
Binary files /dev/null and b/toolkit/content/tests/browser/empty.png differ
diff --git a/toolkit/content/widgets/remote-browser.xml b/toolkit/content/widgets/remote-browser.xml
index 019446689078..c74745cfe4ba 100644
--- a/toolkit/content/widgets/remote-browser.xml
+++ b/toolkit/content/widgets/remote-browser.xml
@@ -322,10 +322,6 @@
               break;
             }
 
-            case "DocumentInserted":
-              this._isSyntheticDocument = data.synthetic;
-              break;
-
             case "FullZoomChange": {
               this._fullZoom = data.value;
               let event = document.createEvent("Events");
diff --git a/toolkit/modules/RemoteWebProgress.jsm b/toolkit/modules/RemoteWebProgress.jsm
index a06140b5f5d8..489629bc4457 100644
--- a/toolkit/modules/RemoteWebProgress.jsm
+++ b/toolkit/modules/RemoteWebProgress.jsm
@@ -198,6 +198,7 @@ RemoteWebProgressManager.prototype = {
         this._browser._imageDocument = null;
         this._browser._mayEnableCharacterEncodingMenu = json.mayEnableCharacterEncodingMenu;
         this._browser._contentPrincipal = json.principal;
+        this._browser._isSyntheticDocument = json.synthetic;
       }
 
       this._callProgressListeners("onLocationChange", webProgress, request, location, flags);