--- a/browser/base/content/test/general/browser_bug553455.js
+++ b/browser/base/content/test/general/browser_bug553455.js
@@ -665,82 +665,101 @@ function test_renotify_installed() {
 
   var triggers = encodeURIComponent(JSON.stringify({
     "XPI": "unsigned.xpi"
   }));
   gBrowser.selectedTab = gBrowser.addTab();
   gBrowser.loadURI(TESTROOT + "installtrigger.html?" + triggers);
 },
 
-/*function test_cancel_restart() {
+function test_cancel_restart() {
+  function complete_install(callback) {
+    let url = TESTROOT + "slowinstall.sjs?continue=true"
+    NetUtil.asyncFetch(url, callback || (() => {}));
+  }
+
   // Wait for the progress notification
   wait_for_notification(PROGRESS_NOTIFICATION, function(aPanel) {
     let notification = aPanel.childNodes[0];
     // Close the notification
     let anchor = document.getElementById("addons-notification-icon");
     anchor.click();
     // Reopen the notification
     anchor.click();
 
     ok(PopupNotifications.isPanelOpen, "Notification should still be open");
     is(PopupNotifications.panel.childNodes.length, 1, "Should be only one notification");
     isnot(notification, aPanel.childNodes[0], "Should have reconstructed the notification UI");
     notification = aPanel.childNodes[0];
     is(notification.id, "addon-progress-notification", "Should have seen the progress notification");
     let button = document.getAnonymousElementByAttribute(notification, "anonid", "cancel");
 
-    // Cancel the download
-    EventUtils.synthesizeMouse(button, 2, 2, {});
+    // Wait for the install to fully cancel
+    let install = notification.notification.options.installs[0];
+    install.addListener({
+      onDownloadCancelled: function() {
+        install.removeListener(this);
 
-    // Notification should have changed to cancelled
-    notification = aPanel.childNodes[0];
-    is(notification.id, "addon-install-cancelled-notification", "Should have seen the cancelled notification");
+        executeSoon(function() {
+          ok(PopupNotifications.isPanelOpen, "Notification should still be open");
+          is(PopupNotifications.panel.childNodes.length, 1, "Should be only one notification");
+          isnot(notification, aPanel.childNodes[0], "Should have reconstructed the notification UI");
+          notification = aPanel.childNodes[0];
+          is(notification.id, "addon-install-cancelled-notification", "Should have seen the cancelled notification");
+
+          // Wait for the install confirmation dialog
+          wait_for_install_dialog(function(aWindow) {
+            // Wait for the complete notification
+            wait_for_notification("addon-install-complete-notification", function(aPanel) {
+              let notification = aPanel.childNodes[0];
+              is(notification.button.label, "Restart Now", "Should have seen the right button");
+              is(notification.getAttribute("label"),
+                 "XPI Test will be installed after you restart " + gApp + ".",
+                 "Should have seen the right message");
 
-    // Wait for the install confirmation dialog
-    wait_for_install_dialog(function(aWindow) {
-      // Wait for the complete notification
-      wait_for_notification("addon-install-complete-notification", function(aPanel) {
-        let notification = aPanel.childNodes[0];
-        is(notification.button.label, "Restart Now", "Should have seen the right button");
-        is(notification.getAttribute("label"),
-           "XPI Test will be installed after you restart " + gApp + ".",
-           "Should have seen the right message");
+              AddonManager.getAllInstalls(function(aInstalls) {
+                is(aInstalls.length, 1, "Should be one pending install");
+                aInstalls[0].cancel();
+
+                Services.perms.remove("example.com", "install");
+                wait_for_notification_close(runNextTest);
+                gBrowser.removeTab(gBrowser.selectedTab);
+              });
+            });
+
+            aWindow.document.documentElement.acceptDialog();
+          });
 
-        AddonManager.getAllInstalls(function(aInstalls) {
-          is(aInstalls.length, 1, "Should be one pending install");
-          aInstalls[0].cancel();
+          // Restart the download
+          EventUtils.synthesizeMouseAtCenter(notification.button, {});
 
-          Services.perms.remove("example.com", "install");
-          wait_for_notification_close(runNextTest);
-          gBrowser.removeTab(gBrowser.selectedTab);
+          // Should be back to a progress notification
+          ok(PopupNotifications.isPanelOpen, "Notification should still be open");
+          is(PopupNotifications.panel.childNodes.length, 1, "Should be only one notification");
+          notification = aPanel.childNodes[0];
+          is(notification.id, "addon-progress-notification", "Should have seen the progress notification");
+
+          complete_install();
         });
-      });
-
-      aWindow.document.documentElement.acceptDialog();
+      }
     });
 
-    // Restart the download
-    EventUtils.synthesizeMouse(notification.button, 20, 10, {});
-
-    // Should be back to a progress notification
-    ok(PopupNotifications.isPanelOpen, "Notification should still be open");
-    is(PopupNotifications.panel.childNodes.length, 1, "Should be only one notification");
-    notification = aPanel.childNodes[0];
-    is(notification.id, "addon-progress-notification", "Should have seen the progress notification");
+    // Cancel the download
+    EventUtils.synthesizeMouseAtCenter(button, {});
   });
 
   var pm = Services.perms;
   pm.add(makeURI("http://example.com/"), "install", pm.ALLOW_ACTION);
 
   var triggers = encodeURIComponent(JSON.stringify({
-    "XPI": "unsigned.xpi"
+    "XPI": "slowinstall.sjs?file=unsigned.xpi"
   }));
   gBrowser.selectedTab = gBrowser.addTab();
   gBrowser.loadURI(TESTROOT + "installtrigger.html?" + triggers);
-},*/
+},
 
 function test_failed_security() {
   Services.prefs.setBoolPref(PREF_INSTALL_REQUIREBUILTINCERTS, false);
 
   setup_redirect({
     "Location": TESTROOT + "unsigned.xpi"
   });
 

--- a/toolkit/mozapps/extensions/test/xpinstall/browser.ini
+++ b/toolkit/mozapps/extensions/test/xpinstall/browser.ini
@@ -19,16 +19,17 @@ support-files =
   redirect.sjs
   restartless.xpi
   signed-no-cn.xpi
   signed-no-o.xpi
   signed-tampered.xpi
   signed-untrusted.xpi
   signed.xpi
   signed2.xpi
+  slowinstall.sjs
   startsoftwareupdate.html
   theme.xpi
   triggerredirect.html
   unsigned.xpi
 
 [browser_auth.js]
 [browser_auth2.js]
 [browser_auth3.js]

new file mode 100644
--- /dev/null
+++ b/toolkit/mozapps/extensions/test/xpinstall/slowinstall.sjs
@@ -0,0 +1,101 @@
+const { classes: Cc, interfaces: Ci, utils: Cu } = Components;
+
+Cu.import("resource://gre/modules/Services.jsm");
+Cu.import("resource://gre/modules/osfile.jsm");
+Cu.import("resource://gre/modules/NetUtil.jsm");
+
+const RELATIVE_PATH = "browser/toolkit/mozapps/extensions/test/xpinstall"
+const NOTIFICATION_TOPIC = "slowinstall-complete";
+
+/**
+ * Helper function to create a JS object representing the url parameters from
+ * the request's queryString.
+ *
+ * @param  aQueryString
+ *         The request's query string.
+ * @return A JS object representing the url parameters from the request's
+ *         queryString.
+ */
+function parseQueryString(aQueryString) {
+  var paramArray = aQueryString.split("&");
+  var regex = /^([^=]+)=(.*)$/;
+  var params = {};
+  for (var i = 0, sz = paramArray.length; i < sz; i++) {
+    var match = regex.exec(paramArray[i]);
+    if (!match)
+      throw "Bad parameter in queryString!  '" + paramArray[i] + "'";
+    params[decodeURIComponent(match[1])] = decodeURIComponent(match[2]);
+  }
+
+  return params;
+}
+
+function handleRequest(aRequest, aResponse) {
+  let id = +getState("ID");
+  setState("ID", "" + (id + 1));
+
+  function LOG(str) {
+    dump("slowinstall.sjs[" + id + "]: " + str + "\n");
+  }
+
+  aResponse.setStatusLine(aRequest.httpVersion, 200, "OK");
+
+  var params = { };
+  if (aRequest.queryString)
+    params = parseQueryString(aRequest.queryString);
+
+  if (params.file) {
+    let xpiFile = "";
+
+    function complete_download() {
+      LOG("Completing download");
+      downloadPaused = false;
+
+      try {
+        // Doesn't seem to be a sane way to read using OS.File and write to an
+        // nsIOutputStream so here we are.
+        let file = Cc["@mozilla.org/file/local;1"].createInstance(Ci.nsIFile);
+        file.initWithPath(xpiFile);
+        let stream = Cc["@mozilla.org/network/file-input-stream;1"].
+                     createInstance(Ci.nsIFileInputStream);
+        stream.init(file, -1, -1, stream.DEFER_OPEN + stream.CLOSE_ON_EOF);
+
+        NetUtil.asyncCopy(stream, aResponse.bodyOutputStream, () => {
+          LOG("Download complete");
+          aResponse.finish();
+        });
+      }
+      catch (e) {
+        LOG("Exception " + e);
+      }
+    }
+
+    let waitForComplete = new Promise(resolve => {
+      function complete() {
+        Services.obs.removeObserver(complete, NOTIFICATION_TOPIC);
+        resolve();
+      }
+
+      Services.obs.addObserver(complete, NOTIFICATION_TOPIC, false);
+    });
+
+    aResponse.processAsync();
+
+    OS.File.getCurrentDirectory().then(dir => {
+      xpiFile = OS.Path.join(dir, ...RELATIVE_PATH.split("/"), params.file);
+      LOG("Starting slow download of " + xpiFile);
+
+      OS.File.stat(xpiFile).then(info => {
+        aResponse.setHeader("Content-Type", "binary/octet-stream");
+        aResponse.setHeader("Content-Length", info.size.toString());
+
+        LOG("Download paused");
+        waitForComplete.then(complete_download);
+      });
+    });
+  }
+  else if (params.continue) {
+    dump("slowinstall.sjs: Received signal to complete all current downloads.\n");
+    Services.obs.notifyObservers(null, NOTIFICATION_TOPIC, null);
+  }
+}