diff --git a/toolkit/mozapps/update/test/unit/test_0030_general.js b/toolkit/mozapps/update/test/unit/test_0030_general.js
index ec8881ca93e..2ae71bfcbb0 100644
--- a/toolkit/mozapps/update/test/unit/test_0030_general.js
+++ b/toolkit/mozapps/update/test/unit/test_0030_general.js
@@ -243,13 +243,13 @@ function initMockIncrementalDownload() {
   var registrar = AUS_Cm.QueryInterface(AUS_Ci.nsIComponentRegistrar);
   gIncrementalDownloadClassID = registrar.contractIDToCID(INC_CONTRACT_ID);
   gIncOldFactory = AUS_Cm.getClassObject(AUS_Cc[INC_CONTRACT_ID],
-                                     AUS_Ci.nsIFactory);
+                                         AUS_Ci.nsIFactory);
   registrar.unregisterFactory(gIncrementalDownloadClassID, gIncOldFactory);
   var components = [IncrementalDownload];
   registrar.registerFactory(gIncrementalDownloadClassID, "",
                             INC_CONTRACT_ID, newFactory);
   gIncOldFactory = AUS_Cm.getClassObject(AUS_Cc[INC_CONTRACT_ID],
-                                     AUS_Ci.nsIFactory);
+                                         AUS_Ci.nsIFactory);
 }
 
 function cleanupMockIncrementalDownload() {
diff --git a/toolkit/mozapps/update/test/unit/test_0082_prompt_unsupportAlreadyNotified.js b/toolkit/mozapps/update/test/unit/test_0082_prompt_unsupportAlreadyNotified.js
new file mode 100644
index 00000000000..6286b2a4173
--- /dev/null
+++ b/toolkit/mozapps/update/test/unit/test_0082_prompt_unsupportAlreadyNotified.js
@@ -0,0 +1,129 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+/**
+ * Test that nsIUpdatePrompt doesn't display UI for showUpdateAvailable for an
+ * unsupported system update when it has already been shown (bug 843497).
+ */
+
+function run_test() {
+  do_test_pending();
+  do_register_cleanup(end_test);
+  removeUpdateDirsAndFiles();
+  setUpdateURLOverride();
+  // The mock XMLHttpRequest is MUCH faster
+  overrideXHR(callHandleEvent);
+  standardInit();
+  // The HTTP server is only used for the mar file downloads which is slow
+  start_httpserver(URL_PATH);
+
+  let registrar = Components.manager.QueryInterface(AUS_Ci.nsIComponentRegistrar);
+  registrar.registerFactory(Components.ID("{1dfeb90a-2193-45d5-9cb8-864928b2af55}"),
+                            "Fake Window Watcher",
+                            "@mozilla.org/embedcomp/window-watcher;1",
+                            WindowWatcherFactory);
+  registrar.registerFactory(Components.ID("{1dfeb90a-2193-45d5-9cb8-864928b2af56}"),
+                            "Fake Window Mediator",
+                            "@mozilla.org/appshell/window-mediator;1",
+                            WindowMediatorFactory);
+
+  logTestInfo("testing nsIUpdatePrompt notifications should not be seen for " +
+              "an unsupported system update when it has already been shown");
+
+  Services.prefs.setBoolPref(PREF_APP_UPDATE_SILENT, false);
+  Services.prefs.setBoolPref(PREF_APP_UPDATE_NOTIFIEDUNSUPPORTED, true);
+  // This preference is used to determine when the background update check has
+  // completed since a successful check will clear the preference.
+  Services.prefs.setIntPref(PREF_APP_UPDATE_BACKGROUNDERRORS, 1);
+
+  gResponseBody = getRemoteUpdatesXMLString("  <update type=\"major\" " +
+                                            "name=\"Unsupported Update\" " +
+                                            "unsupported=\"true\" " +
+                                            "detailsURL=\"" + URL_HOST +
+                                            "\"></update>\n");
+  gAUS.notify(null);
+  do_execute_soon(check_test);
+}
+
+function check_test() {
+  if (Services.prefs.prefHasUserValue(PREF_APP_UPDATE_BACKGROUNDERRORS)) {
+    do_execute_soon(check_test);
+    return;
+  }
+  do_check_true(true);
+  do_test_finished();
+}
+
+function end_test() {
+  let registrar = Components.manager.QueryInterface(AUS_Ci.nsIComponentRegistrar);
+  registrar.unregisterFactory(Components.ID("{1dfeb90a-2193-45d5-9cb8-864928b2af55}"),
+                              WindowWatcherFactory);
+  registrar.unregisterFactory(Components.ID("{1dfeb90a-2193-45d5-9cb8-864928b2af56}"),
+                              WindowMediatorFactory);
+  cleanUp();
+}
+
+// Callback function used by the custom XMLHttpRequest implementation to
+// call the nsIDOMEventListener's handleEvent method for onload.
+function callHandleEvent() {
+  gXHR.status = 400;
+  gXHR.responseText = gResponseBody;
+  try {
+    var parser = AUS_Cc["@mozilla.org/xmlextras/domparser;1"].
+                 createInstance(AUS_Ci.nsIDOMParser);
+    gXHR.responseXML = parser.parseFromString(gResponseBody, "application/xml");
+  }
+  catch(e) {
+  }
+  var e = { target: gXHR };
+  gXHR.onload(e);
+}
+
+function check_showUpdateAvailable() {
+  do_throw("showUpdateAvailable should not have called openWindow!");
+}
+
+var WindowWatcher = {
+  openWindow: function(aParent, aUrl, aName, aFeatures, aArgs) {
+    check_showUpdateAvailable();
+  },
+
+  QueryInterface: function(aIID) {
+    if (aIID.equals(AUS_Ci.nsIWindowWatcher) ||
+        aIID.equals(AUS_Ci.nsISupports))
+      return this;
+
+    throw AUS_Cr.NS_ERROR_NO_INTERFACE;
+  }
+}
+
+var WindowWatcherFactory = {
+  createInstance: function createInstance(aOuter, aIID) {
+    if (aOuter != null)
+      throw AUS_Cr.NS_ERROR_NO_AGGREGATION;
+    return WindowWatcher.QueryInterface(aIID);
+  }
+};
+
+var WindowMediator = {
+  getMostRecentWindow: function(aWindowType) {
+    return null;
+  },
+
+  QueryInterface: function(aIID) {
+    if (aIID.equals(AUS_Ci.nsIWindowMediator) ||
+        aIID.equals(AUS_Ci.nsISupports))
+      return this;
+
+    throw AUS_Cr.NS_ERROR_NO_INTERFACE;
+  }
+}
+
+var WindowMediatorFactory = {
+  createInstance: function createInstance(aOuter, aIID) {
+    if (aOuter != null)
+      throw AUS_Cr.NS_ERROR_NO_AGGREGATION;
+    return WindowMediator.QueryInterface(aIID);
+  }
+};
diff --git a/toolkit/mozapps/update/test/unit/xpcshell.ini b/toolkit/mozapps/update/test/unit/xpcshell.ini
index 3aa25fb8407..3eba7ad685f 100644
--- a/toolkit/mozapps/update/test/unit/xpcshell.ini
+++ b/toolkit/mozapps/update/test/unit/xpcshell.ini
@@ -20,6 +20,9 @@ tail =
 [test_0081_prompt_uiAlreadyOpen.js]
 skip-if = toolkit == "gonk"
 reason = custom nsIUpdatePrompt
+[test_0082_prompt_unsupportAlreadyNotified.js]
+skip-if = toolkit == "gonk"
+reason = custom nsIUpdatePrompt
 ; Tests that require the updater binary
 [include:xpcshell_updater.ini]
 skip-if = os == 'android'