diff --git a/dom/notification/test/mochitest/mochitest.ini b/dom/notification/test/mochitest/mochitest.ini
index 81c0a716a4f3..a7e58061299e 100644
--- a/dom/notification/test/mochitest/mochitest.ini
+++ b/dom/notification/test/mochitest/mochitest.ini
@@ -16,4 +16,3 @@ scheme = https
 [test_notification_storage.html]
 [test_bug931307.html]
 [test_notification_tag.html]
-fail-if = fission
diff --git a/dom/notification/test/mochitest/test_notification_tag.html b/dom/notification/test/mochitest/test_notification_tag.html
index 084966fbc527..a763b7c25d33 100644
--- a/dom/notification/test/mochitest/test_notification_tag.html
+++ b/dom/notification/test/mochitest/test_notification_tag.html
@@ -20,104 +20,125 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=782211
 </pre>
 <script type="text/javascript">
   /* eslint-disable mozilla/use-chromeutils-generateqi */
-  const MOCK_CID = SpecialPowers.wrap(SpecialPowers.Components).ID(
-    "{dbe37e64-d9a3-402c-8d8a-0826c619f7ad}"
-  );
-  const ALERTS_SERVICE_CONTRACT_ID = "@mozilla.org/alerts-service;1";
 
-  var mockAlertsService = {
-    showAlert(alert, alertListener) {
-      notificationsCreated.push(alert.name);
-      if (notificationsCreated.length == 3) {
-        checkNotifications();
-      }
-    },
+  // The mock is not a general purpose mock, but is specific for this test.
+  // It is always registered in the parent process using LoadChromeScript by
+  // the MockAlertsService below, to allow this to work regardless of whether
+  // the frames from different origins live in the same process or in different
+  // processes (with Fission), since the default content-process alerts service
+  // relays messages to the parent process.
+  function mockServicesChromeScript() {
+    const MOCK_CID = Components.ID("{dbe37e64-d9a3-402c-8d8a-0826c619f7ad}");
+    const ALERTS_SERVICE_CONTRACT_ID = "@mozilla.org/alerts-service;1";
 
-    showAlertNotification(
-      imageUrl,
-      title,
-      text,
-      textClickable,
-      cookie,
-      alertListener,
-      name,
-      dir,
-      lang,
-      data
-    ) {
-      this.showAlert({ name });
-    },
+    var notificationsCreated = [];
 
-    QueryInterface(aIID) {
-      if (
-        SpecialPowers.wrap(aIID).equals(SpecialPowers.Ci.nsISupports) ||
-        SpecialPowers.wrap(aIID).equals(SpecialPowers.Ci.nsIAlertsService)
+    const mockAlertsService = {
+      showAlert(alert, alertListener) {
+        notificationsCreated.push(alert.name);
+        if (notificationsCreated.length == 3) {
+          // notifications created by the test1 origin
+          var test1notifications = [];
+          // notifications created by the test2 origin
+          var test2notifications = [];
+          for (var i = 0; i < notificationsCreated.length; i++) {
+            var notificationName = notificationsCreated[i];
+            if (notificationName.includes("test1")) {
+              test1notifications.push(notificationsCreated[i]);
+            } else if (notificationName.includes("test2")) {
+              test2notifications.push(notificationsCreated[i]);
+            }
+          }
+
+          is(
+            test1notifications.length,
+            2,
+            "2 notifications should be created by test1.example.org:80 origin."
+          );
+          is(
+            test1notifications[0],
+            test1notifications[1],
+            "notification names should be identical."
+          );
+          is(
+            test2notifications.length,
+            1,
+            "1 notification should be created by test2.example.org:80 origin."
+          );
+
+          // Register original alerts service.
+          registrar.unregisterFactory(MOCK_CID, this);
+
+          sendAsyncMessage("mock-alert-service:unregistered");
+        }
+      },
+
+      showAlertNotification(
+        imageUrl,
+        title,
+        text,
+        textClickable,
+        cookie,
+        alertListener,
+        name,
+        dir,
+        lang,
+        data
       ) {
-        return this;
-      }
-      throw SpecialPowers.Components.results.NS_ERROR_NO_INTERFACE;
-    },
+        this.showAlert({ name });
+      },
 
-    createInstance(aOuter, aIID) {
-      if (aOuter != null) {
-        throw SpecialPowers.Components.results.NS_ERROR_NO_AGGREGATION;
+      QueryInterface(aIID) {
+        if (aIID.equals(Ci.nsISupports) || aIID.equals(Ci.nsIAlertsService)) {
+          return this;
+        }
+        throw Components.Exception("", Cr.NS_ERROR_NO_INTERFACE);
+      },
+
+      createInstance(aOuter, aIID) {
+        if (aOuter != null) {
+          throw Components.Exception("", Cr.NS_ERROR_NO_AGGREGATION);
+        }
+        return this.QueryInterface(aIID);
+      },
+    };
+
+    const registrar = Components.manager.QueryInterface(Ci.nsIComponentRegistrar);
+
+    registrar.registerFactory(
+      MOCK_CID,
+      "alerts service",
+      ALERTS_SERVICE_CONTRACT_ID,
+      mockAlertsService
+    );
+
+    const { sendAsyncMessage } = this;
+
+    sendAsyncMessage("mock-alert-service:registered");
+  }
+
+  const MockAlertsService = {
+    async register() {
+      if (this._chromeScript) {
+        throw new Error("MockAlertsService already registered");
       }
-      return this.QueryInterface(aIID);
+      this._chromeScript = SpecialPowers.loadChromeScript(
+        mockServicesChromeScript
+      );
+      await this._chromeScript.promiseOneMessage("mock-alert-service:registered");
+    },
+    async unregistered() {
+      await this._chromeScript.promiseOneMessage(
+        "mock-alert-service:unregistered"
+      );
     },
   };
-  mockAlertsService = SpecialPowers.wrapCallbackObject(mockAlertsService);
-
-  var notificationsCreated = [];
-  function checkNotifications() {
-    // notifications created by the test1 origin
-    var test1notifications = [];
-    // notifications created by the test2 origin
-    var test2notifications = [];
-    for (var i = 0; i < notificationsCreated.length; i++) {
-      var notificationName = notificationsCreated[i];
-      if (notificationName.includes("test1")) {
-        test1notifications.push(notificationsCreated[i]);
-      } else if (notificationName.includes("test2")) {
-        test2notifications.push(notificationsCreated[i]);
-      }
-    }
-
-    is(
-      test1notifications.length,
-      2,
-      "2 notifications should be created by test1.example.org:80 origin."
-    );
-    is(
-      test1notifications[0],
-      test1notifications[1],
-      "notification names should be identical."
-    );
-    is(
-      test2notifications.length,
-      1,
-      "1 notification should be created by test2.example.org:80 origin."
-    );
-
-    // Register original alerts service.
-    SpecialPowers.wrap(SpecialPowers.Components)
-      .manager.QueryInterface(SpecialPowers.Ci.nsIComponentRegistrar)
-      .unregisterFactory(MOCK_CID, mockAlertsService);
-
-    SimpleTest.finish();
-  }
 
   if (window.Notification) {
     SimpleTest.waitForExplicitFinish();
 
-    function showNotifications() {
-      SpecialPowers.wrap(SpecialPowers.Components)
-        .manager.QueryInterface(SpecialPowers.Ci.nsIComponentRegistrar)
-        .registerFactory(
-          MOCK_CID,
-          "alerts service",
-          ALERTS_SERVICE_CONTRACT_ID,
-          mockAlertsService
-        );
+    async function showNotifications() {
+      await MockAlertsService.register();
 
       // Load two frames with the same origin that create notification with the same tag.
       // Both pages should generate notifications with the same name, and thus the second
@@ -130,6 +151,10 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=782211
       // The notification name should be different and thus no notifications should be replaced.
       frames.crossDomain.location.href =
         "http://test2.example.org:80/tests/dom/notification/test/mochitest/create_notification.html";
+
+      await MockAlertsService.unregistered();
+
+      SimpleTest.finish();
     }
 
     SpecialPowers.pushPrefEnv(