diff --git a/browser/actors/PluginParent.sys.mjs b/browser/actors/PluginParent.sys.mjs
index ba4b82fa3455..fa93c1d5ab90 100644
--- a/browser/actors/PluginParent.sys.mjs
+++ b/browser/actors/PluginParent.sys.mjs
@@ -178,11 +178,20 @@ export class PluginParent extends JSWindowActorParent {
       buttons.push(submitButton);
     }
 
+    // Add the "learn more" link.
+    let learnMoreLink = {
+      supportPage: "plugin-crashed-notificationbar",
+      label: lazy.gNavigatorBundle.GetStringFromName(
+        "crashedpluginsMessage.learnMore"
+      ),
+    };
+    buttons.push(learnMoreLink);
+
     let messageString = lazy.gNavigatorBundle.formatStringFromName(
       "crashedpluginsMessage.title",
       [report.pluginName]
     );
-    notification = notificationBox.appendNotification(
+    notificationBox.appendNotification(
       "plugin-crashed",
       {
         label: messageString,
@@ -191,21 +200,5 @@ export class PluginParent extends JSWindowActorParent {
       },
       buttons
     );
-
-    // Add the "learn more" link.
-    let link = notification.ownerDocument.createXULElement("label", {
-      is: "text-link",
-    });
-    link.setAttribute(
-      "value",
-      lazy.gNavigatorBundle.GetStringFromName("crashedpluginsMessage.learnMore")
-    );
-    let crashurl = Services.urlFormatter.formatURLPref("app.support.baseURL");
-    crashurl += "plugin-crashed-notificationbar";
-    link.href = crashurl;
-    // Append a blank text node to make sure we don't put
-    // the link right next to the end of the message text.
-    notification.messageText.appendChild(new Text(" "));
-    notification.messageText.appendChild(link);
   }
 }
diff --git a/browser/base/content/browser-data-submission-info-bar.js b/browser/base/content/browser-data-submission-info-bar.js
index 896542796c54..26d9affb2920 100644
--- a/browser/base/content/browser-data-submission-info-bar.js
+++ b/browser/base/content/browser-data-submission-info-bar.js
@@ -40,7 +40,7 @@ var gDataNotificationInfoBar = {
     return gNotificationBox.getNotificationWithValue(name);
   },
 
-  _displayDataPolicyInfoBar(request) {
+  async _displayDataPolicyInfoBar(request) {
     if (this._getDataReportingNotification()) {
       return;
     }
@@ -59,7 +59,7 @@ var gDataNotificationInfoBar = {
     ];
 
     this._log.info("Creating data reporting policy notification.");
-    gNotificationBox.appendNotification(
+    await gNotificationBox.appendNotification(
       this._DATA_REPORTING_NOTIFICATION,
       {
         label: {
diff --git a/browser/base/content/browser.js b/browser/base/content/browser.js
index b899b27878f8..4197d33be2d3 100644
--- a/browser/base/content/browser.js
+++ b/browser/base/content/browser.js
@@ -1106,7 +1106,7 @@ const gStoragePressureObserver = {
     }
     messageFragment.appendChild(message);
 
-    gNotificationBox.appendNotification(
+    await gNotificationBox.appendNotification(
       NOTIFICATION_VALUE,
       {
         label: messageFragment,
@@ -1122,7 +1122,7 @@ const gStoragePressureObserver = {
 };
 
 var gPopupBlockerObserver = {
-  handleEvent(aEvent) {
+  async handleEvent(aEvent) {
     if (aEvent.originalTarget != gBrowser.selectedBrowser) {
       return;
     }
@@ -1158,23 +1158,31 @@ var gPopupBlockerObserver = {
 
         let notificationBox = gBrowser.getNotificationBox();
         let notification =
-          notificationBox.getNotificationWithValue("popup-blocked");
+          notificationBox.getNotificationWithValue("popup-blocked") ||
+          (await this.notificationPromise);
         if (notification) {
           notification.label = label;
         } else {
           const image = "chrome://browser/skin/notification-icons/popup.svg";
           const priority = notificationBox.PRIORITY_INFO_MEDIUM;
-          notificationBox.appendNotification(
-            "popup-blocked",
-            { label, image, priority },
-            [
-              {
-                "l10n-id": "popup-warning-button",
-                popup: "blockedPopupOptions",
-                callback: null,
-              },
-            ]
-          );
+          try {
+            this.notificationPromise = notificationBox.appendNotification(
+              "popup-blocked",
+              { label, image, priority },
+              [
+                {
+                  "l10n-id": "popup-warning-button",
+                  popup: "blockedPopupOptions",
+                  callback: null,
+                },
+              ]
+            );
+            await this.notificationPromise;
+          } catch (err) {
+            console.warn(err);
+          } finally {
+            this.notificationPromise = null;
+          }
         }
       }
 
@@ -1402,7 +1410,7 @@ var gKeywordURIFixup = {
     let asciiHost = fixedURI.asciiHost;
 
     let onLookupCompleteListener = {
-      onLookupComplete(request, record, status) {
+      async onLookupComplete(request, record, status) {
         let browserRef = weakBrowser.get();
         if (!Components.isSuccessCode(status) || !browserRef) {
           return;
@@ -1457,7 +1465,7 @@ var gKeywordURIFixup = {
             },
           },
         ];
-        let notification = notificationBox.appendNotification(
+        let notification = await notificationBox.appendNotification(
           "keyword-uri-fixup",
           {
             label: message,
@@ -3334,22 +3342,9 @@ function PageProxyClickHandler(aEvent) {
  * us via async messaging.
  */
 var BrowserOnClick = {
-  ignoreWarningLink(reason, blockedInfo, browsingContext) {
-    let triggeringPrincipal =
-      blockedInfo.triggeringPrincipal ||
-      _createNullPrincipalFromTabUserContextId();
-
-    // Allow users to override and continue through to the site,
-    // but add a notify bar as a reminder, so that they don't lose
-    // track after, e.g., tab switching.
-    // Note that we have to use the passed URI info and can't just
-    // rely on the document URI, because the latter contains
-    // additional query parameters that should be stripped.
-    browsingContext.fixupAndLoadURIString(blockedInfo.uri, {
-      triggeringPrincipal,
-      flags: Ci.nsIWebNavigation.LOAD_FLAGS_BYPASS_CLASSIFIER,
-    });
-
+  async ignoreWarningLink(reason, blockedInfo, browsingContext) {
+    // Add a notify bar before allowing the user to continue through to the
+    // site, so that they don't lose track after, e.g., tab switching.
     // We can't use browser.contentPrincipal which is principal of about:blocked
     // Create one from uri with current principal origin attributes
     let principal = Services.scriptSecurityManager.createContentPrincipal(
@@ -3424,7 +3419,20 @@ var BrowserOnClick = {
       // provide a URL endpoint for these reports.
     }
 
-    SafeBrowsingNotificationBox.show(title, buttons);
+    await SafeBrowsingNotificationBox.show(title, buttons);
+
+    // Allow users to override and continue through to the site.
+    // Note that we have to use the passed URI info and can't just
+    // rely on the document URI, because the latter contains
+    // additional query parameters that should be stripped.
+    let triggeringPrincipal =
+      blockedInfo.triggeringPrincipal ||
+      _createNullPrincipalFromTabUserContextId();
+
+    browsingContext.fixupAndLoadURIString(blockedInfo.uri, {
+      triggeringPrincipal,
+      flags: Ci.nsIWebNavigation.LOAD_FLAGS_BYPASS_CLASSIFIER,
+    });
   },
 };
 
@@ -9059,7 +9067,7 @@ var PanicButtonNotifier = {
 
 const SafeBrowsingNotificationBox = {
   _currentURIBaseDomain: null,
-  show(title, buttons) {
+  async show(title, buttons) {
     let uri = gBrowser.currentURI;
 
     // start tracking host so that we know when we leave the domain
@@ -9080,7 +9088,7 @@ const SafeBrowsingNotificationBox = {
       notificationBox.removeNotification(previousNotification);
     }
 
-    let notification = notificationBox.appendNotification(
+    let notification = await notificationBox.appendNotification(
       value,
       {
         label: title,
diff --git a/browser/base/content/test/general/browser_datachoices_notification.js b/browser/base/content/test/general/browser_datachoices_notification.js
index fd57140b6851..5a13feedd177 100644
--- a/browser/base/content/test/general/browser_datachoices_notification.js
+++ b/browser/base/content/test/general/browser_datachoices_notification.js
@@ -176,6 +176,7 @@ add_task(async function test_single_window() {
   // Wait for the infobar to be displayed.
   triggerInfoBar(10 * 1000);
   await alertShownPromise;
+  await promiseNextTick();
 
   Assert.equal(
     gNotificationBox.allNotifications.length,
diff --git a/browser/base/content/test/general/browser_refreshBlocker.js b/browser/base/content/test/general/browser_refreshBlocker.js
index cbeb9efdbcd1..637f47a545e8 100644
--- a/browser/base/content/test/general/browser_refreshBlocker.js
+++ b/browser/base/content/test/general/browser_refreshBlocker.js
@@ -187,21 +187,23 @@ add_task(async function test_can_update_notification() {
       // should have fired, so the notification should be visible.
       let notificationBox = gBrowser.getNotificationBox(browser);
       let notification = notificationBox.currentNotification;
+      let [redirectLabel, refreshLabel] = await document.l10n.formatValues([
+        { id: "refresh-blocked-redirect-label" },
+        { id: "refresh-blocked-refresh-label" },
+      ]);
 
-      let message = notification.messageText.querySelector("span");
       is(
-        message.dataset.l10nId,
-        "refresh-blocked-redirect-label",
+        notification.messageText.textContent.trim(),
+        redirectLabel,
         "Should be showing the redirect message"
       );
 
       // Next, attempt a refresh
       await attemptFakeRefresh(browser, false);
 
-      message = notification.messageText.querySelector("span");
       is(
-        message.dataset.l10nId,
-        "refresh-blocked-refresh-label",
+        notification.messageText.textContent.trim(),
+        refreshLabel,
         "Should be showing the refresh message"
       );
     }
diff --git a/browser/base/content/test/notificationbox/browser_notificationbar_telemetry.js b/browser/base/content/test/notificationbox/browser_notificationbar_telemetry.js
index 7810d4022d70..c775f2f9e983 100644
--- a/browser/base/content/test/notificationbox/browser_notificationbar_telemetry.js
+++ b/browser/base/content/test/notificationbox/browser_notificationbar_telemetry.js
@@ -31,15 +31,16 @@ add_task(async function showNotification() {
 
   verifyTelemetry("initial", 0, 0, 0, 0, 0, 0);
 
-  let notif3 = box3.appendNotification("infobar-testtwo-value", {
+  let notif3 = await box3.appendNotification("infobar-testtwo-value", {
     label: "Message for tab 3",
     priority: box3.PRIORITY_INFO_HIGH,
     telemetry: TELEMETRY_BASE + "testtwo",
   });
+  await notif3.updateComplete;
 
   verifyTelemetry("first notification", 0, 0, 0, 0, 0, 1);
 
-  let notif1 = box1.appendNotification(
+  let notif1 = await box1.appendNotification(
     "infobar-testone-value",
     {
       label: "Message for tab 1",
@@ -60,6 +61,7 @@ add_task(async function showNotification() {
       },
     ]
   );
+  await notif1.updateComplete;
   verifyTelemetry("second notification", 0, 0, 0, 0, 0, 1);
 
   await BrowserTestUtils.switchTab(gBrowser, tab1);
@@ -89,7 +91,7 @@ add_task(async function showNotification() {
   notif3.dismiss();
   verifyTelemetry("dismiss notification for box 3", 1, 1, 1, 1, 1, 1, 1);
 
-  let notif4 = box1.appendNotification(
+  let notif4 = await box1.appendNotification(
     "infobar-testtwo-value",
     {
       label: "Additional message for tab 1",
@@ -103,13 +105,14 @@ add_task(async function showNotification() {
       },
     ]
   );
+  await notif4.updateComplete;
   verifyTelemetry("show first filtered notification", 2, 1, 1, 1, 1, 1, 1);
 
   notif4.buttonContainer.lastElementChild.click();
   notif4.dismiss();
   verifyTelemetry("dismiss first filtered notification", 2, 1, 1, 1, 1, 1, 1);
 
-  let notif5 = box1.appendNotification(
+  let notif5 = await box1.appendNotification(
     "infobar-testtwo-value",
     {
       label: "Dimissed additional message for tab 1",
@@ -123,13 +126,14 @@ add_task(async function showNotification() {
       },
     ]
   );
+  await notif5.updateComplete;
   verifyTelemetry("show second filtered notification", 2, 1, 1, 1, 1, 1, 1);
 
   notif5.buttonContainer.lastElementChild.click();
   notif5.dismiss();
   verifyTelemetry("dismiss second filtered notification", 2, 1, 1, 1, 2, 1, 1);
 
-  let notif6 = box1.appendNotification(
+  let notif6 = await box1.appendNotification(
     "infobar-testtwo-value",
     {
       label: "Dimissed additional message for tab 1",
@@ -144,6 +148,7 @@ add_task(async function showNotification() {
       },
     ]
   );
+  await notif6.updateComplete;
   verifyTelemetry("show third filtered notification", 2, 1, 1, 1, 2, 1, 1);
 
   notif6.buttonContainer.lastElementChild.click();
diff --git a/browser/base/content/test/notificationbox/browser_tabnotificationbox_switch_tabs.js b/browser/base/content/test/notificationbox/browser_tabnotificationbox_switch_tabs.js
index f00916c77357..230336606b7d 100644
--- a/browser/base/content/test/notificationbox/browser_tabnotificationbox_switch_tabs.js
+++ b/browser/base/content/test/notificationbox/browser_tabnotificationbox_switch_tabs.js
@@ -32,9 +32,9 @@ function assertNotificationBoxShown(reason, browser) {
   is(selectedViewName, name, `Box is shown ${reason}`);
 }
 
-function createNotification({ browser, label, value, priority }) {
+async function createNotification({ browser, label, value, priority }) {
   let notificationBox = gBrowser.getNotificationBox(browser);
-  let notification = notificationBox.appendNotification(value, {
+  let notification = await notificationBox.appendNotification(value, {
     label,
     priority: notificationBox[priority],
   });
@@ -52,7 +52,7 @@ add_task(async function testNotificationInBackgroundTab() {
     gBrowser.selectedTab = firstTab;
     assertNotificationBoxHidden("initial first tab");
 
-    createNotification({
+    await createNotification({
       browser,
       label: "My notification body",
       value: "test-notification",
@@ -69,7 +69,7 @@ add_task(async function testNotificationInActiveTab() {
   await BrowserTestUtils.withNewTab("about:blank", async browser => {
     ok(!gBrowser.readNotificationBox(browser), "No notifications for new tab");
 
-    createNotification({
+    await createNotification({
       browser,
       label: "Notification!",
       value: "test-notification",
@@ -108,7 +108,7 @@ add_task(async function testNotificationMultipleTabs() {
   assertNotificationBoxHidden("after open", browserTwo);
   assertNotificationBoxHidden("after open", browserThree);
 
-  createNotification({
+  await createNotification({
     browser: browserTwo,
     label: "Test blank",
     value: "blank",
@@ -121,7 +121,7 @@ add_task(async function testNotificationMultipleTabs() {
   assertNotificationBoxHidden("hidden create", browserTwo);
   assertNotificationBoxHidden("other create", browserThree);
 
-  createNotification({
+  await createNotification({
     browser: browserThree,
     label: "Test active tab",
     value: "active",
diff --git a/browser/base/content/test/plugins/browser_enable_DRM_prompt.js b/browser/base/content/test/plugins/browser_enable_DRM_prompt.js
index e77455ddd0d7..bc861806b194 100644
--- a/browser/base/content/test/plugins/browser_enable_DRM_prompt.js
+++ b/browser/base/content/test/plugins/browser_enable_DRM_prompt.js
@@ -36,6 +36,11 @@ add_task(async function test_drm_prompt_shows_for_toplevel() {
     // Turn off EME and Widevine CDM.
     Services.prefs.setBoolPref("media.eme.enabled", false);
     Services.prefs.setBoolPref("media.gmp-widevinecdm.enabled", false);
+    let notificationShownPromise = BrowserTestUtils.waitForNotificationBar(
+      gBrowser,
+      browser,
+      "drmContentDisabled"
+    );
 
     // Have content request access to Widevine, UI should drop down to
     // prompt user to enable DRM.
@@ -64,7 +69,9 @@ add_task(async function test_drm_prompt_shows_for_toplevel() {
 
     // Verify the UI prompt showed.
     let box = gBrowser.getNotificationBox(browser);
+    await notificationShownPromise;
     let notification = box.currentNotification;
+    await notification.updateComplete;
 
     ok(notification, "Notification should be visible");
     is(
@@ -152,6 +159,11 @@ add_task(async function test_drm_prompt_shows_for_cross_origin_iframe() {
     // Turn off EME and Widevine CDM.
     Services.prefs.setBoolPref("media.eme.enabled", false);
     Services.prefs.setBoolPref("media.gmp-widevinecdm.enabled", false);
+    let notificationShownPromise = BrowserTestUtils.waitForNotificationBar(
+      gBrowser,
+      browser,
+      "drmContentDisabled"
+    );
 
     // Have content request access to Widevine, UI should drop down to
     // prompt user to enable DRM.
@@ -198,7 +210,9 @@ add_task(async function test_drm_prompt_shows_for_cross_origin_iframe() {
 
     // Verify the UI prompt showed.
     let box = gBrowser.getNotificationBox(browser);
+    await notificationShownPromise;
     let notification = box.currentNotification;
+    await notification.updateComplete;
 
     ok(notification, "Notification should be visible");
     is(
diff --git a/browser/base/content/test/plugins/browser_globalplugin_crashinfobar.js b/browser/base/content/test/plugins/browser_globalplugin_crashinfobar.js
index 483c2b403262..38ac3864c024 100644
--- a/browser/base/content/test/plugins/browser_globalplugin_crashinfobar.js
+++ b/browser/base/content/test/plugins/browser_globalplugin_crashinfobar.js
@@ -54,7 +54,7 @@ add_task(async function () {
         "Correct priority."
       );
       is(
-        notification.messageText.textContent,
+        notification.messageText.textContent.trim(),
         "The GlobalTestPlugin plugin has crashed.",
         "Correct message."
       );
diff --git a/browser/components/BrowserGlue.sys.mjs b/browser/components/BrowserGlue.sys.mjs
index 4290ec7f1cef..b74ee0dc91ac 100644
--- a/browser/components/BrowserGlue.sys.mjs
+++ b/browser/components/BrowserGlue.sys.mjs
@@ -3736,12 +3736,12 @@ BrowserGlue.prototype = {
    * Show the notificationBox for a locked places database.
    */
   _showPlacesLockedNotificationBox:
-    function BG__showPlacesLockedNotificationBox() {
+    async function BG__showPlacesLockedNotificationBox() {
       var win = lazy.BrowserWindowTracker.getTopWindow();
       var buttons = [{ supportPage: "places-locked" }];
 
       var notifyBox = win.gBrowser.getNotificationBox();
-      var notification = notifyBox.appendNotification(
+      var notification = await notifyBox.appendNotification(
         "places-locked",
         {
           label: { "l10n-id": "places-locked-prompt" },
@@ -4616,7 +4616,7 @@ BrowserGlue.prototype = {
     ];
 
     const notifyBox = win.gBrowser.getNotificationBox();
-    const notification = notifyBox.appendNotification(
+    const notification = await notifyBox.appendNotification(
       "startup-restore-session-suggestion",
       {
         label: messageFragment,
diff --git a/browser/components/protocolhandler/test/browser/browser_registerProtocolHandler_notification.js b/browser/components/protocolhandler/test/browser/browser_registerProtocolHandler_notification.js
index e96ed7060eea..e2598ae2817f 100644
--- a/browser/components/protocolhandler/test/browser/browser_registerProtocolHandler_notification.js
+++ b/browser/components/protocolhandler/test/browser/browser_registerProtocolHandler_notification.js
@@ -40,16 +40,9 @@ add_task(async function () {
     "info",
     "We expect this notification to have the type of 'info'."
   );
-
-  // Make sure the CSS is fully loaded...
-  ok(
-    await TestUtils.waitForCondition(
-      () =>
-        notification.ownerGlobal.getComputedStyle(
-          notification.messageImage,
-          "::after"
-        ).backgroundImage == 'url("chrome://global/skin/icons/info-filled.svg")'
-    ),
+  is(
+    notification.messageImage.getAttribute("src"),
+    "chrome://global/skin/icons/info-filled.svg",
     "We expect this notification to have an icon."
   );
 
diff --git a/devtools/client/responsive/test/browser/browser_screenshot_button_warning.js b/devtools/client/responsive/test/browser/browser_screenshot_button_warning.js
index 46d0371f9858..f7940b3cbf17 100644
--- a/devtools/client/responsive/test/browser/browser_screenshot_button_warning.js
+++ b/devtools/client/responsive/test/browser/browser_screenshot_button_warning.js
@@ -46,7 +46,7 @@ addRDMTask(
     const notificationEl = box.currentNotification;
     ok(notificationEl, "Notification should be visible");
     is(
-      notificationEl.messageText.textContent,
+      notificationEl.messageText.textContent.trim(),
       "The device pixel ratio was reduced to 1 as the resulting image was too large",
       "The expected warning was displayed"
     );
diff --git a/dom/media/doctor/test/browser/browser_decoderDoctor.js b/dom/media/doctor/test/browser/browser_decoderDoctor.js
index ece4b453eb5d..3586515208fb 100644
--- a/dom/media/doctor/test/browser/browser_decoderDoctor.js
+++ b/dom/media/doctor/test/browser/browser_decoderDoctor.js
@@ -112,7 +112,7 @@ async function test_decoder_doctor_notification(
           label = await document.l10n.formatValue(label.l10nId);
         }
         if (isLink) {
-          let link = notification.messageText.querySelector("a");
+          let link = notification.supportLinkEls[0];
           if (link) {
             // Seems to be a Windows specific quirk, but without this
             // mutation observer the notification.messageText.textContent
@@ -126,13 +126,13 @@ async function test_decoder_doctor_notification(
           }
         }
         is(
-          notification.messageText.textContent,
-          notificationMessage + (isLink && label ? ` ${label}` : ""),
+          notification.messageText.textContent.trim(),
+          notificationMessage,
           "notification message should match expectation"
         );
 
         let button = notification.buttonContainer.querySelector("button");
-        let link = notification.messageText.querySelector("a");
+        let link = notification.supportLinkEls[0];
         if (!label) {
           ok(!button, "There should not be a button");
           ok(!link, "There should not be a link");
@@ -141,7 +141,7 @@ async function test_decoder_doctor_notification(
 
         if (isLink) {
           ok(!button, "There should not be a button");
-          is(link.innerText, label, `notification link should be '${label}'`);
+          is(link.textContent, label, `notification link should be '${label}'`);
           ok(
             !link.hasAttribute("accesskey"),
             "notification link should not have accesskey"
diff --git a/testing/firefox-ui/tests/functional/safebrowsing/test_notification.py b/testing/firefox-ui/tests/functional/safebrowsing/test_notification.py
index 4bc8d1219cff..2489542381e3 100644
--- a/testing/firefox-ui/tests/functional/safebrowsing/test_notification.py
+++ b/testing/firefox-ui/tests/functional/safebrowsing/test_notification.py
@@ -123,7 +123,7 @@ class TestSafeBrowsingNotificationBar(WindowManagerMixin, MarionetteTestCase):
             message = notification_box.find_element(
                 By.CSS_SELECTOR, "notification-message[value=blocked-badware-page]"
             )
-            button = message.get_property("closeButton")
+            button = message.get_property("closeButtonEl")
             button.click()
 
             Wait(self.marionette, timeout=self.marionette.timeout.page_load).until(
diff --git a/toolkit/content/jar.mn b/toolkit/content/jar.mn
index 0462eaea14c1..09ab9f1fb6e0 100644
--- a/toolkit/content/jar.mn
+++ b/toolkit/content/jar.mn
@@ -106,6 +106,7 @@ toolkit.jar:
    content/global/elements/moz-toggle.css      (widgets/moz-toggle/moz-toggle.css)
    content/global/elements/moz-toggle.mjs      (widgets/moz-toggle/moz-toggle.mjs)
    content/global/elements/named-deck.js       (widgets/named-deck.js)
+   content/global/elements/infobar.css         (widgets/infobar.css)
    content/global/elements/notificationbox.js  (widgets/notificationbox.js)
    content/global/elements/panel.js            (widgets/panel.js)
    content/global/elements/panel-item.css      (widgets/panel-list/panel-item.css)
diff --git a/toolkit/content/tests/chrome/test_bug457632.xhtml b/toolkit/content/tests/chrome/test_bug457632.xhtml
index 7f94eaef09a0..12267c1decfd 100644
--- a/toolkit/content/tests/chrome/test_bug457632.xhtml
+++ b/toolkit/content/tests/chrome/test_bug457632.xhtml
@@ -28,14 +28,14 @@ function completeAnimation(nextTest) {
   setTimeout(completeAnimation, 50, nextTest);
 }
 
-function test() {
+async function test() {
   SimpleTest.waitForExplicitFinish();
   gNotificationBox = new MozElements.NotificationBox(e => {
     document.getElementById("nb").appendChild(e);
   });
 
   is(gNotificationBox.allNotifications.length, 0, "There should be no initial notifications");
-  gNotificationBox.appendNotification("notification1",
+  await gNotificationBox.appendNotification("notification1",
     { label: "Test notification", priority: gNotificationBox.PRIORITY_INFO_LOW });
   is(gNotificationBox.allNotifications.length, 1, "Notification exists while animating in");
   let notification = gNotificationBox.getNotificationWithValue("notification1");
@@ -46,7 +46,7 @@ function test() {
 }
 
 // Tests that a notification that is fully animated in gets removed immediately
-function test1() {
+async function test1() {
   let notification = gNotificationBox.getNotificationWithValue("notification1");
   gNotificationBox.removeNotification(notification);
   notification = gNotificationBox.getNotificationWithValue("notification1");
@@ -59,8 +59,8 @@ function test1() {
 }
 
 // Tests that a notification that is animating in gets removed immediately
-function test2() {
-  let notification = gNotificationBox.appendNotification("notification2",
+async function test2() {
+  let notification = await gNotificationBox.appendNotification("notification2",
     { label: "Test notification", priority: gNotificationBox.PRIORITY_INFO_LOW });
   gNotificationBox.removeNotification(notification);
   notification = gNotificationBox.getNotificationWithValue("notification2");
@@ -74,10 +74,10 @@ function test2() {
 }
 
 // Tests that a background notification goes away immediately
-function test3() {
-  let notification = gNotificationBox.appendNotification("notification3",
+async function test3() {
+  let notification = await gNotificationBox.appendNotification("notification3",
     { label: "Test notification", priority: gNotificationBox.PRIORITY_INFO_LOW });
-  let notification2 = gNotificationBox.appendNotification("notification4",
+  let notification2 = await gNotificationBox.appendNotification("notification4",
     { label: "Test notification", priority: gNotificationBox.PRIORITY_INFO_LOW });
   is(gNotificationBox.allNotifications.length, 2, "Test 3 should show 2 notifications present");
   gNotificationBox.removeNotification(notification);
@@ -96,10 +96,10 @@ function test3() {
 }
 
 // Tests that a foreground notification hiding a background one goes away
-function test4() {
-  let notification = gNotificationBox.appendNotification("notification5",
+async function test4() {
+  let notification = await gNotificationBox.appendNotification("notification5",
     { label: "Test notification", priority: gNotificationBox.PRIORITY_INFO_LOW });
-  let notification2 = gNotificationBox.appendNotification("notification6",
+  let notification2 = await gNotificationBox.appendNotification("notification6",
     { label: "Test notification", priority: gNotificationBox.PRIORITY_INFO_LOW });
   gNotificationBox.removeNotification(notification2);
   notification2 = gNotificationBox.getNotificationWithValue("notification6");
@@ -118,10 +118,10 @@ function test4() {
 }
 
 // Tests that removeAllNotifications gets rid of everything
-function test5() {
-  let notification = gNotificationBox.appendNotification("notification7",
+async function test5() {
+  let notification = await gNotificationBox.appendNotification("notification7",
     { label: "Test notification", priority: gNotificationBox.PRIORITY_INFO_LOW });
-  let notification2 = gNotificationBox.appendNotification("notification8",
+  let notification2 = await gNotificationBox.appendNotification("notification8",
     { label: "Test notification", priority: gNotificationBox.PRIORITY_INFO_LOW });
   gNotificationBox.removeAllNotifications();
   notification = gNotificationBox.getNotificationWithValue("notification7");
@@ -131,7 +131,7 @@ function test5() {
   ok(!gNotificationBox.currentNotification, "Test 5 said there was still a current notification");
   is(gNotificationBox.allNotifications.length, 0, "Test 5 should show 0 notifications present");
 
-  gNotificationBox.appendNotification("notification9",
+  await gNotificationBox.appendNotification("notification9",
     { label: "Test notification", priority: gNotificationBox.PRIORITY_INFO_LOW });
 
   // Wait for the notificaton to finish displaying
@@ -139,7 +139,7 @@ function test5() {
 }
 
 // Tests whether removing an already removed notification doesn't break things
-function test6() {
+async function test6() {
   let notification = gNotificationBox.getNotificationWithValue("notification9");
   ok(notification, "Test 6 should have an initial notification");
   gNotificationBox.removeNotification(notification);
@@ -147,7 +147,7 @@ function test6() {
 
   ok(!gNotificationBox.currentNotification, "Test 6 shouldn't be any current notification");
   is(gNotificationBox.allNotifications.length, 0, "Test 6 allNotifications.length should be 0");
-  notification = gNotificationBox.appendNotification("notification10",
+  notification = await gNotificationBox.appendNotification("notification10",
     { label: "Test notification", priority: gNotificationBox.PRIORITY_INFO_LOW });
   is(notification, gNotificationBox.currentNotification, "Test 6 should have made the current notification");
   gNotificationBox.removeNotification(notification);
diff --git a/toolkit/content/tests/chrome/test_bug509732.xhtml b/toolkit/content/tests/chrome/test_bug509732.xhtml
index c886a05fd134..7e340322b296 100644
--- a/toolkit/content/tests/chrome/test_bug509732.xhtml
+++ b/toolkit/content/tests/chrome/test_bug509732.xhtml
@@ -20,7 +20,7 @@
 var gNotificationBox;
 
 // Tests that a notification that is added in an hidden box didn't throw the animation
-function test() {
+async function test() {
   SimpleTest.waitForExplicitFinish();
   gNotificationBox = new MozElements.NotificationBox(e => {
     document.getElementById("nb").appendChild(e);
@@ -28,7 +28,7 @@ function test() {
 
   is(gNotificationBox.allNotifications.length, 0, "There should be no initial notifications");
 
-  gNotificationBox.appendNotification("notification1",
+  await gNotificationBox.appendNotification("notification1",
     { label: "Test notification", priority: gNotificationBox.PRIORITY_INFO_LOW });
 
   is(gNotificationBox.allNotifications.length, 1, "Notification exists");
diff --git a/toolkit/content/tests/chrome/test_notificationbox.xhtml b/toolkit/content/tests/chrome/test_notificationbox.xhtml
index 483adec77a30..8de985175a29 100644
--- a/toolkit/content/tests/chrome/test_notificationbox.xhtml
+++ b/toolkit/content/tests/chrome/test_notificationbox.xhtml
@@ -138,18 +138,15 @@ function testtag_notification_eventCallback(expectedEvents, ntf, testName)
 var tests =
 [
   {
-    test(nb, ntf) {
-      ntf = nb.appendNotification("mutable", {
+    async test(nb, ntf) {
+      ntf = await nb.appendNotification("mutable", {
           label: "Original",
           priority: nb.PRIORITY_INFO_LOW,
         }, testtag_notificationbox_buttons);
 
       ntf.label = "Changed string";
-      SimpleTest.is(ntf.messageText.textContent, "Changed string", "set notification label with string");
-
-      ntf.label = { "l10n-id": "foo-bar" };
-      const message = ntf.messageText.querySelector("span");
-      SimpleTest.is(message.dataset.l10nId, "foo-bar", "set notification label with l10n id");
+      await ntf.updateComplete;
+      SimpleTest.is(ntf.messageText.textContent.trim(), "Changed string", "set notification label with string");
       return ntf;
     },
     result(nb, ntf) {
@@ -162,8 +159,8 @@ var tests =
   label attribute set correctly.
   */
   {
-    test(nb, ntf) {
-      ntf = nb.appendNotification("note", {
+    async test(nb, ntf) {
+      ntf = await nb.appendNotification("note", {
         label: "Notification",
         image: "happy.png",
         priority: nb.PRIORITY_INFO_LOW,
@@ -183,8 +180,8 @@ var tests =
     Ensures that buttons created with the "l10n-id" parameter have
     their "l10n-id" assigned correctly.
     */
-    test(nb, ntf) {
-      ntf = nb.appendNotification("note", {
+    async test(nb, ntf) {
+      ntf = await nb.appendNotification("note", {
         label: "Notification",
         image: "happy.png",
         priority: nb.PRIORITY_INFO_LOW,
@@ -200,9 +197,9 @@ var tests =
     }
   },
   {
-    test(nb, ntf) {
+    async test(nb, ntf) {
       // append a new notification
-      ntf = nb.appendNotification("note", {
+      ntf = await nb.appendNotification("note", {
           label: "Notification", 
           image: "happy.png",
           priority: nb.PRIORITY_INFO_LOW,
@@ -235,9 +232,9 @@ var tests =
     }
   },
   {
-    test(nb, ntf) {
+    async test(nb, ntf) {
       // append a new notification, but now with an event callback
-      ntf = nb.appendNotification("note", {
+      ntf = await nb.appendNotification("note", {
           label: "Notification", 
           image: "happy.png",
           priority: nb.PRIORITY_INFO_LOW,
@@ -265,8 +262,8 @@ var tests =
     }
   },
   {
-    test(nb, ntf) {
-      ntf = nb.appendNotification("note", {
+    async test(nb, ntf) {
+      ntf = await nb.appendNotification("note", {
           label: "Notification", 
           image: "happy.png",
           priority: nb.PRIORITY_INFO_MEDIUM,
@@ -299,8 +296,8 @@ var tests =
     }
   },
   {
-    test(nb, ntf) {
-      ntf = nb.appendNotification("note", {
+    async test(nb, ntf) {
+      ntf = await nb.appendNotification("note", {
           label: "Notification", 
           image: "happy.png",
           priority: nb.PRIORITY_WARNING_LOW,
@@ -321,13 +318,13 @@ var tests =
   },
   {
     repeat: true,
-    test(nb, arr) {
+    async test(nb, arr) {
       var idx = arr[0];
       var ntf = arr[1];
       switch (idx) {
         case 1:
           // append a new notification
-          ntf = nb.appendNotification("note", {
+          ntf = await nb.appendNotification("note", {
               label: "Notification",
               image: "happy.png",
               priority: nb.PRIORITY_INFO_LOW,
@@ -373,9 +370,9 @@ var tests =
     }
   },
   {
-    test(nb, ntf) {
+    async test(nb, ntf) {
       // append another notification
-      ntf = nb.appendNotification("note", {
+      ntf = await nb.appendNotification("note", {
           label: "Notification",
           image: "happy.png",
           priority: nb.PRIORITY_INFO_MEDIUM,
@@ -402,8 +399,8 @@ var tests =
     }
   },
   {
-    test(nb, ntf) {
-      ntf = nb.appendNotification("note", {
+    async test(nb, ntf) {
+      ntf = await nb.appendNotification("note", {
           label: "Notification",
           image: "happy.png",
           priority: nb.PRIORITY_INFO_HIGH,
@@ -419,8 +416,8 @@ var tests =
     }
   },
   {
-    test(nb, ntf) {
-      ntf = nb.appendNotification("note", {
+    async test(nb, ntf) {
+      ntf = await nb.appendNotification("note", {
           label: "Notification",
           image: "happy.png",
           priority: nb.PRIORITY_INFO_LOW,
@@ -437,7 +434,7 @@ var tests =
       SimpleTest.is(button.localName, "button", "button is a button");
       SimpleTest.ok(!button.href, "button href is not set");
 
-      let link = ntf.messageText.lastElementChild;
+      let link = ntf.querySelector(".notification-link");
       SimpleTest.is(link.localName, "label", "link is a label");
       SimpleTest.is(link.href, "about:mozilla", "link href is correct");
 
@@ -445,9 +442,9 @@ var tests =
     }
   },
   {
-    test(nb, ntf) {
+    async test(nb, ntf) {
       // append a new notification
-      ntf = nb.appendNotification("note", {
+      ntf = await nb.appendNotification("note", {
           label: "Notification",
           image: "happy.png",
           priority: nb.PRIORITY_INFO_LOW,
@@ -473,20 +470,21 @@ var tests =
     }
   },
   {
-    test(nb, ntf) {
-      ntf = nb.appendNotification("note", {
+    async test(nb, ntf) {
+      ntf = await nb.appendNotification("note", {
         label: "Notification",
         image: "happy.png",
         priority: nb.PRIORITY_INFO_LOW,
         eventCallback: notification_eventCallback,
       }, testtag_notificationbox_supportpage);
+      await ntf.updateComplete;
       SimpleTest.is(ntf && ntf.localName == NOTIFICATION_LOCAL_NAME, true, "append support page notification");
       return ntf;
     },
     result(nb, ntf) {
       testtag_notificationbox_State(nb, "append link with callback", ntf, 1);
 
-      let link = ntf.messageText.firstElementChild;
+      let link = ntf.querySelector(".notification-link");
       SimpleTest.is(link.localName, "a", "link 1 is an anchor");
       SimpleTest.is(link.dataset.l10nId, "moz-support-link-text", "link 1 Fluent ID is set");
       SimpleTest.ok(link.href.endsWith("/test1"), "link 1 href is set");
@@ -512,20 +510,20 @@ var tests =
     }
   },
   {
-    test(nb, unused) {
+    async test(nb, unused) {
       // add a number of notifications and check that they are added in order
-      nb.appendNotification("4", { label: "Four", priority: nb.PRIORITY_INFO_HIGH },
+      await nb.appendNotification("4", { label: "Four", priority: nb.PRIORITY_INFO_HIGH },
                             testtag_notificationbox_buttons);
-      nb.appendNotification("7", { label: "Seven", priority: nb.PRIORITY_WARNING_HIGH },
+      await nb.appendNotification("7", { label: "Seven", priority: nb.PRIORITY_WARNING_HIGH },
                             testtag_notificationbox_buttons);
-      nb.appendNotification("2", { label: "Two", priority: nb.PRIORITY_INFO_LOW });
-      nb.appendNotification("8", { label: "Eight", priority: nb.PRIORITY_CRITICAL_LOW });
-      nb.appendNotification("5", { label: "Five", priority: nb.PRIORITY_WARNING_LOW });
-      nb.appendNotification("6", { label: "Six", priority: nb.PRIORITY_WARNING_HIGH });
-      nb.appendNotification("1", { label: "One", priority: nb.PRIORITY_INFO_LOW });
-      nb.appendNotification("9", { label: "Nine", priority: nb.PRIORITY_CRITICAL_MEDIUM });
-      let ntf = nb.appendNotification("10", { label: "Ten", priority: nb.PRIORITY_CRITICAL_HIGH });
-      nb.appendNotification("3", { label: "Three", priority: nb.PRIORITY_INFO_MEDIUM });
+      await nb.appendNotification("2", { label: "Two", priority: nb.PRIORITY_INFO_LOW });
+      await nb.appendNotification("8", { label: "Eight", priority: nb.PRIORITY_CRITICAL_LOW });
+      await nb.appendNotification("5", { label: "Five", priority: nb.PRIORITY_WARNING_LOW });
+      await nb.appendNotification("6", { label: "Six", priority: nb.PRIORITY_WARNING_HIGH });
+      await nb.appendNotification("1", { label: "One", priority: nb.PRIORITY_INFO_LOW });
+      await nb.appendNotification("9", { label: "Nine", priority: nb.PRIORITY_CRITICAL_MEDIUM });
+      let ntf = await nb.appendNotification("10", { label: "Ten", priority: nb.PRIORITY_CRITICAL_HIGH });
+      await nb.appendNotification("3", { label: "Three", priority: nb.PRIORITY_INFO_MEDIUM });
       return ntf;
     },
     result(nb, ntf) {
@@ -558,16 +556,16 @@ var tests =
     }
   },
   {
-    test(nb, ntf) {
+    async test(nb, ntf) {
       var exh = false;
       try {
-        nb.appendNotification("no", { label: "no", priority: -1 });
+        await nb.appendNotification("no", { label: "no", priority: -1 });
       } catch (ex) { exh = true; }
       SimpleTest.is(exh, true, "appendNotification priority too low");
 
       exh = false;
       try {
-        nb.appendNotification("no", { label: "no", priority: 11 });
+        await nb.appendNotification("no", { label: "no", priority: 11 });
       } catch (ex) { exh = true; }
       SimpleTest.is(exh, true, "appendNotification priority too high");
 
@@ -579,8 +577,8 @@ var tests =
 
 var appendPriorityTests = [
   {
-    test(nb, priority) {
-      let ntf = nb.appendNotification("note", {
+    async test(nb, priority) {
+      let ntf = await nb.appendNotification("note", {
           label: "Notification", 
           image: "happy.png",
           priority,
@@ -598,9 +596,9 @@ var appendPriorityTests = [
       nb.removeCurrentNotification();
       return priority;
     },
-    result(nb, priority) {
+    async result(nb, priority) {
       if (priority == nb.PRIORITY_CRITICAL_HIGH) {
-        let ntf = nb.appendNotification("note", {
+        let ntf = await nb.appendNotification("note", {
             label: "Notification", 
             image: "happy.png",
             priority: nb.PRIORITY_INFO_LOW,
@@ -623,7 +621,7 @@ function testtag_notificationbox_State(nb, testid, expecteditem, expectedcount)
 
 function testtag_notification_State(nb, ntf, testid, label, value, image, priority)
 {
-  is(ntf.messageText.textContent, label, testid + " notification label");
+  is(ntf.messageText.textContent.trim(), label, testid + " notification label");
   is(ntf.getAttribute("value"), value, testid + " notification value");
   is(ntf.priority, priority, testid + " notification priority");
 
@@ -654,7 +652,7 @@ function testtag_notification_State(nb, ntf, testid, label, value, image, priori
     critical: "chrome://global/skin/icons/error.svg",
   };
   let icon = icons[type];
-  is(getComputedStyle(ntf.messageImage, "::after").backgroundImage, `url("${icon}")`, "notification image is set");
+  is(ntf.messageImage.src, icon, "notification image is set");
 }
 
 function checkPopupTest(nb, ntf)
@@ -697,7 +695,7 @@ function checkPopupClosed()
  * function as its arg. The result function may also return a value which
  * will be passed to the next repetition or the next test in the array.
  */
-function runTimedTests(tests, idx, element, arg)
+async function runTimedTests(tests, idx, element, arg)
 {
   if (idx >= 0 && "result" in tests[idx])
     arg = tests[idx].result(element, arg);
@@ -707,7 +705,7 @@ function runTimedTests(tests, idx, element, arg)
     idx++;
 
   if (idx < tests.length) {
-    var result = tests[idx].test(element, arg);
+    let result = await tests[idx].test(element, arg);
     setTimeout(runTimedTestsWait, 50, tests, idx, element, result);
   }
 }
diff --git a/toolkit/content/widgets/infobar.css b/toolkit/content/widgets/infobar.css
new file mode 100644
index 000000000000..ee811818b524
--- /dev/null
+++ b/toolkit/content/widgets/infobar.css
@@ -0,0 +1,99 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+:host(.infobar) {
+  --info-bar-background-color: light-dark(var(--color-white), rgb(66, 65, 77));
+  --info-bar-text-color: light-dark(var(--color-gray-100), var(--color-gray-05));
+  position: relative;
+
+  &::before {
+    content: "";
+    display: block;
+    width: 2px;
+    position: absolute;
+    top: 0;
+    inset-inline-start: 0;
+    height: 100%;
+    border-start-start-radius: 4px;
+    border-end-start-radius: 4px;
+  }
+
+  .container {
+    /* Don't let lwthemes set a text-shadow. */
+    text-shadow: none;
+    padding-block: 3px;
+    align-items: center;
+  }
+
+  .content {
+    gap: 0 12px;
+    height: fit-content;
+  }
+
+  .close {
+    margin-block: 4px;
+    margin-inline-start: 8px;
+    background-size: 12px;
+    height: 24px;
+    width: 24px;
+    align-self: flex-start;
+  }
+}
+
+@media (prefers-contrast) {
+  :host(.infobar)::before {
+    background-color: CanvasText;
+  }
+}
+
+@media not (prefers-contrast) {
+  :host(.infobar) {
+    box-shadow: 0 1px 2px rgba(58, 57, 68, 0.1);
+    background-color: var(--info-bar-background-color);
+    color: var(--info-bar-text-color);
+
+    &::before {
+      background-image: linear-gradient(0deg, #9059ff 0%, #ff4aa2 52.08%, #ffbd4f 100%);
+    }
+  }
+}
+
+:host([message-bar-type=infobar]:first-of-type) {
+  margin-top: 4px;
+}
+
+:host([message-bar-type=infobar]) {
+  margin: 0 4px 4px;
+}
+
+::slotted(.notification-button-container) {
+  gap: 8px;
+  display: inline-flex;
+}
+
+::slotted(.text-link) {
+  margin: 0 !important;
+}
+
+img.inline-icon {
+  /* Align inline icon images in the message content */
+  vertical-align: middle;
+  /* Ensure they get the right fill color. */
+  -moz-context-properties: fill;
+  fill: currentColor;
+}
+
+strong {
+  font-weight: var(--font-weight-bold);
+}
+
+/* type="system" infobar styles */
+
+:host([type=system]) .icon {
+  display: none;
+}
+
+:host([type=system]) .content {
+  margin-inline-start: 0;
+}
diff --git a/toolkit/content/widgets/message-bar.css b/toolkit/content/widgets/message-bar.css
index 616b62f2118c..eddc5a3ae6c6 100644
--- a/toolkit/content/widgets/message-bar.css
+++ b/toolkit/content/widgets/message-bar.css
@@ -13,10 +13,6 @@
   --close-icon-size: 28px;
 }
 
-:host([message-bar-type=infobar]) {
-  --close-icon-size: 28px;
-}
-
 :host {
   --message-bar-background-color: var(--in-content-box-info-background);
   --message-bar-text-color: var(--in-content-text-color);
@@ -28,7 +24,7 @@
   --message-bar-icon-url: var(--warn-icon-url);
 }
 
-:host([type=success]:not([message-bar-type=infobar])) {
+:host([type=success]) {
   --message-bar-icon-url: var(--success-icon-url);
 }
 
@@ -125,26 +121,23 @@
   min-height: auto;
   width: var(--close-icon-size);
   height: var(--close-icon-size);
-  margin: 0;
   padding: 0;
   flex-shrink: 0;
+  margin: 4px 8px;
+  background-size: 12px;
 }
 
 @media (prefers-contrast) {
   :host {
     border-color: CanvasText;
   }
-
-  .container.infobar::before {
-    background-color: CanvasText;
-  }
 }
 
 @media not (prefers-contrast) {
   /* MessageBar colors by message type */
   /* Colors from: https://design.firefox.com/photon/components/message-bars.html#type-specific-style */
 
-  :host([type=warning]:not([message-bar-type=infobar])) {
+  :host([type=warning]) {
     /* Ensure colors within the bar are adjusted and controls are readable */
     color-scheme: light;
 
@@ -158,7 +151,7 @@
     --close-fill-color: var(--message-bar-text-color);
   }
 
-  :host([type=success]:not([message-bar-type=infobar])) {
+  :host([type=success]) {
     /* Ensure colors within the bar are adjusted and controls are readable */
     color-scheme: light;
 
@@ -170,7 +163,7 @@
     --in-content-button-background-active: var(--green-80);
   }
 
-  :host([type=error]:not([message-bar-type=infobar])) {
+  :host([type=error]) {
     --message-bar-background-color: var(--red-60);
     --message-bar-text-color: #ffffff;
 
@@ -191,15 +184,6 @@
     color: rgb(226,40,80);
   }
 
-  .container.infobar {
-    box-shadow: 0 1px 2px rgba(58, 57, 68, 0.1);
-    background: var(--in-content-page-background);
-  }
-
-  .container.infobar::before {
-    background-image: linear-gradient(0deg, #9059ff 0%, #ff4aa2 52.08%, #ffbd4f 100%);
-  }
-
   .close {
     fill: var(--close-fill-color);
   }
@@ -207,10 +191,6 @@
   @media (prefers-color-scheme: dark) {
     /* Don't set the background in prefers-contrast mode or macOS can end up
      * with black on black text. */
-    .container.infobar {
-      background: rgb(66,65,77);
-    }
-
     :host([type=info]) .icon {
       color: rgb(128,235,255);
     }
@@ -225,81 +205,6 @@
   }
 }
 
-:host([message-bar-type=infobar]:first-of-type) {
-  margin-top: 4px;
-}
-
-:host([message-bar-type=infobar]) {
-  margin: 0 4px 4px;
-}
-
-.container.infobar {
-  /* Don't let lwthemes set a text-shadow. */
-  text-shadow: none;
-  padding: 0;
-}
-
-.container.infobar::before {
-  content: "";
-  display: block;
-  width: 2px;
-  position: absolute;
-  top: 0;
-  inset-inline-start: 0;
-  height: 100%;
-  border-start-start-radius: 4px;
-  border-end-start-radius: 4px;
-}
-
-.container.infobar {
-  align-items: flex-start;
-}
-
-/* Infobars styling. */
-.notification-content {
-  margin: 0;
-  margin-inline-start: 8px;
-  padding-block: 3px;
-}
-
-.notification-message {
-  padding-block: 8px;
-  margin-inline-end: 20px;
-  /* Prevent misalignment issue in the infobar on Linux  */
-  @media (-moz-platform: linux) {
-    padding-block-start: 6px;
-  }
-}
-
-.notification-button-container,
-.notification-message {
-  display: inline-block;
-  vertical-align: middle;
-}
-
-.container.infobar > .notification-content > .notification-message img.inline-icon {
-  /* Align inline icon images in the message content */
-  vertical-align: middle;
-  /* Ensure they get the right fill color. */
-  -moz-context-properties: fill;
-  fill: currentColor;
-}
-
-.close {
-  margin: 4px 8px;
-  background-size: 12px;
-}
-
-.notification-button {
-  align-items: center;
-  margin: 0 4px;
-}
-
-.notification-button:first-of-type {
-  /* When the buttons wrap to their own line we want to match the 8px on the message. */
-  margin-inline-start: 0;
-}
-
 strong {
   font-weight: 600;
 }
@@ -308,20 +213,6 @@ strong {
   cursor: pointer;
 }
 
-.infobar > .icon {
-  padding: 0;
-  margin: 10px 0;
-}
-
-.infobar > .icon,
-:host([type=system]) .notification-content {
-  margin-inline-start: 16px;
-}
-
-:host([type=system]) .icon {
-  display: none;
-}
-
 @keyframes spin {
   from { transform: rotate(0); }
   to { transform: rotate(360deg); }
diff --git a/toolkit/content/widgets/moz-message-bar/moz-message-bar.css b/toolkit/content/widgets/moz-message-bar/moz-message-bar.css
index e80916d711d3..6d3500998244 100644
--- a/toolkit/content/widgets/moz-message-bar/moz-message-bar.css
+++ b/toolkit/content/widgets/moz-message-bar/moz-message-bar.css
@@ -177,26 +177,27 @@
 
   :host([type=warning]) {
     --message-bar-background-color: var(--color-background-warning);
+
+    .icon {
+      --message-bar-icon-color: var(--icon-color-warning);
+    }
   }
 
   :host([type=success]) {
     --message-bar-background-color: var(--color-background-success);
+
+    .icon {
+      --message-bar-icon-color: var(--icon-color-success);
+    }
   }
 
-  :host([type=error]) {
+  :host([type=error]),
+  :host([type=critical]) {
     --message-bar-background-color: var(--color-background-critical);
-  }
 
-  :host([type=success]) .icon {
-    --message-bar-icon-color: var(--icon-color-success);
-  }
-
-  :host([type=warning]) .icon {
-    --message-bar-icon-color: var(--icon-color-warning);
-  }
-
-  :host([type=error]) .icon {
-    --message-bar-icon-color: var(--icon-color-critical);
+    .icon {
+      --message-bar-icon-color: var(--icon-color-critical);
+    }
   }
 
   .close {
diff --git a/toolkit/content/widgets/moz-message-bar/moz-message-bar.mjs b/toolkit/content/widgets/moz-message-bar/moz-message-bar.mjs
index acd3638e2351..58f41c28e4a9 100644
--- a/toolkit/content/widgets/moz-message-bar/moz-message-bar.mjs
+++ b/toolkit/content/widgets/moz-message-bar/moz-message-bar.mjs
@@ -22,6 +22,10 @@ const messageTypeToIconData = {
     iconSrc: "chrome://global/skin/icons/error.svg",
     l10nId: "moz-message-bar-icon-error",
   },
+  critical: {
+    iconSrc: "chrome://global/skin/icons/error.svg",
+    l10nId: "moz-message-bar-icon-error",
+  },
 };
 
 /**
@@ -33,6 +37,8 @@ const messageTypeToIconData = {
  * @property {string} heading - The heading of the message.
  * @property {string} message - The message text.
  * @property {boolean} dismissable - Whether or not the element is dismissable.
+ * @property {string} messageL10nId - l10n ID for the message.
+ * @property {string} messageL10nArgs - Any args needed for the message l10n ID.
  * @fires message-bar:close
  *  Custom event indicating that message bar was closed.
  *  @fires message-bar:user-dismissed
@@ -44,6 +50,7 @@ export default class MozMessageBar extends MozLitElement {
     actionsSlotEl: "slot[name=actions]",
     actionsEl: ".actions",
     closeButtonEl: "button.close",
+    supportLinkSlotEl: "slot[name=support-link]",
   };
 
   static properties = {
@@ -51,6 +58,8 @@ export default class MozMessageBar extends MozLitElement {
     heading: { type: String },
     message: { type: String },
     dismissable: { type: Boolean },
+    messageL10nId: { type: String },
+    messageL10nArgs: { type: String },
   };
 
   constructor() {
@@ -75,6 +84,10 @@ export default class MozMessageBar extends MozLitElement {
     this.dispatchEvent(new CustomEvent("message-bar:close"));
   }
 
+  get supportLinkEls() {
+    return this.supportLinkSlotEl.assignedElements();
+  }
+
   iconTemplate() {
     let iconData = messageTypeToIconData[this.type];
     if (iconData) {
@@ -126,7 +139,15 @@ export default class MozMessageBar extends MozLitElement {
             <div class="text-content">
               ${this.headingTemplate()}
               <div>
-                <span class="message">${ifDefined(this.message)}</span>
+                <span
+                  class="message"
+                  data-l10n-id=${ifDefined(this.messageL10nId)}
+                  data-l10n-args=${ifDefined(
+                    JSON.stringify(this.messageL10nArgs)
+                  )}
+                >
+                  ${this.message}
+                </span>
                 <span class="link">
                   <slot name="support-link"></slot>
                 </span>
diff --git a/toolkit/content/widgets/notificationbox.js b/toolkit/content/widgets/notificationbox.js
index 889001a0cfed..f23fb03a7466 100644
--- a/toolkit/content/widgets/notificationbox.js
+++ b/toolkit/content/widgets/notificationbox.js
@@ -144,7 +144,7 @@
      *
      * @return The <notification> element that is shown.
      */
-    appendNotification(aType, aNotification, aButtons) {
+    async appendNotification(aType, aNotification, aButtons) {
       if (
         aNotification.priority < this.PRIORITY_SYSTEM ||
         aNotification.priority > this.PRIORITY_CRITICAL_HIGH
@@ -157,12 +157,19 @@
       MozXULElement.insertFTLIfNeeded("toolkit/global/notification.ftl");
 
       // Create the Custom Element and connect it to the document immediately.
-      var newitem;
+      let newitem;
       if (!aNotification.notificationIs) {
         if (!customElements.get("notification-message")) {
           // There's some weird timing stuff when this element is created at
           // script load time, we don't need it until now anyway so be lazy.
-          createNotificationMessageElement();
+          // Wrapped in a try/catch to handle rare cases where we start creating
+          // a notification but then the window gets closed/goes away.
+          try {
+            await createNotificationMessageElement();
+          } catch (err) {
+            console.warn(err);
+            throw err;
+          }
         }
         newitem = document.createElement("notification-message");
         newitem.setAttribute("message-bar-type", "infobar");
@@ -182,8 +189,10 @@
         this.stack.append(newitem);
       }
 
-      // Custom notification classes may not have the messageText property.
-      if (newitem.messageText) {
+      if (newitem.localName === "notification-message" && aNotification.label) {
+        newitem.label = aNotification.label;
+      } else if (newitem.messageText) {
+        // Custom notification classes may not have the messageText property.
         // Can't use instanceof in case this was created from a different document:
         if (
           aNotification.label &&
@@ -241,6 +250,10 @@
       newitem.style.top = "100%";
       newitem.style.marginTop = "-15px";
       newitem.style.opacity = "0";
+
+      // Ensure the DOM has been created for the Lit-based notification-message
+      // element so that we add the .animated class + it animates as expected.
+      await newitem.updateComplete;
       this._showNotification(newitem, true);
 
       // Fire event for accessibility APIs
@@ -607,53 +620,56 @@
 
   customElements.define("notification", MozElements.Notification);
 
-  function createNotificationMessageElement() {
-    // Get a reference to MessageBarElement from a created element so the import
-    // gets handled automatically if needed.
-    class NotificationMessage extends document.createElement("message-bar")
-      .constructor {
+  async function createNotificationMessageElement() {
+    await window.ensureCustomElements("moz-message-bar");
+    let MozMessageBar = customElements.get("moz-message-bar");
+    class NotificationMessage extends MozMessageBar {
+      static queries = {
+        ...MozMessageBar.queries,
+        messageText: ".message",
+        messageImage: ".icon",
+      };
+
       constructor() {
         super();
         this.persistence = 0;
         this.priority = 0;
         this.timeout = 0;
         this.telemetry = null;
+        this.dismissable = true;
         this._shown = false;
+
+        this.addEventListener("click", this);
+        this.addEventListener("command", this);
       }
 
       connectedCallback() {
-        this.toggleAttribute("dismissable", true);
-        this.closeButton.classList.add("notification-close");
+        super.connectedCallback();
+        this.#setStyles();
 
-        this.container = this.shadowRoot.querySelector(".container");
-        this.container.classList.add("infobar");
+        this.classList.add("infobar");
         this.setAlertRole();
 
-        let messageContent = this.shadowRoot.querySelector(".content");
-        messageContent.classList.add("notification-content");
-
-        // Remove the <slot>, API surface is `set label()` and `setButtons()`.
-        messageContent.textContent = "";
-
-        // A 'label' allows screen readers to detect the text of the alert.
-        this.messageText = document.createElement("label");
-        this.messageText.classList.add("notification-message");
         this.buttonContainer = document.createElement("span");
         this.buttonContainer.classList.add("notification-button-container");
-
-        this.messageImage = this.shadowRoot.querySelector(".icon");
-
-        messageContent.append(this.messageText, this.buttonContainer);
-        this.shadowRoot.addEventListener("click", this);
-        this.shadowRoot.addEventListener("command", this);
+        this.buttonContainer.setAttribute("slot", "actions");
+        this.appendChild(this.buttonContainer);
       }
 
       disconnectedCallback() {
+        super.disconnectedCallback();
         if (this.eventCallback) {
           this.eventCallback("disconnected");
         }
       }
 
+      #setStyles() {
+        let style = document.createElement("link");
+        style.rel = "stylesheet";
+        style.href = "chrome://global/content/elements/infobar.css";
+        this.renderRoot.append(style);
+      }
+
       _doTelemetry(type) {
         if (
           this.telemetry &&
@@ -686,10 +702,10 @@
       setAlertRole() {
         // Wait a little for this to render before setting the role for more
         // consistent alerts to screen readers.
-        this.container.removeAttribute("role");
+        this.removeAttribute("role");
         window.requestAnimationFrame(() => {
           window.requestAnimationFrame(() => {
-            this.container.setAttribute("role", "alert");
+            this.setAttribute("role", "alert");
           });
         });
       }
@@ -736,18 +752,10 @@
        */
       set label(value) {
         if (value && typeof value == "object" && "l10n-id" in value) {
-          const message = document.createElement("span");
-          document.l10n.setAttributes(
-            message,
-            value["l10n-id"],
-            value["l10n-args"]
-          );
-          while (this.messageText.firstChild) {
-            this.messageText.firstChild.remove();
-          }
-          this.messageText.appendChild(message);
+          this.messageL10nId = value["l10n-id"];
+          this.messageL10nArgs = value["l10n-args"];
         } else {
-          this.messageText.textContent = value;
+          this.message = value;
         }
         this.setAlertRole();
       }
@@ -777,7 +785,11 @@
               "button",
               button.is ? { is: button.is } : {}
             );
-            buttonElem.classList.add("notification-button", "small-button");
+            buttonElem.classList.add(
+              "notification-button",
+              "small-button",
+              "footer-button"
+            );
 
             if (button.primary) {
               buttonElem.classList.add("primary");
@@ -794,7 +806,8 @@
           }
 
           if (link) {
-            this.messageText.append(new Text(" "), buttonElem);
+            buttonElem.setAttribute("slot", "support-link");
+            this.appendChild(buttonElem);
           } else {
             this.buttonContainer.appendChild(buttonElem);
           }
@@ -811,6 +824,8 @@
         super.dismiss();
       }
     }
-    customElements.define("notification-message", NotificationMessage);
+    if (!customElements.get("notification-message")) {
+      customElements.define("notification-message", NotificationMessage);
+    }
   }
 }
diff --git a/toolkit/themes/shared/design-system/tokens-shared.css b/toolkit/themes/shared/design-system/tokens-shared.css
index a2b103aae06b..86b14f1daa68 100644
--- a/toolkit/themes/shared/design-system/tokens-shared.css
+++ b/toolkit/themes/shared/design-system/tokens-shared.css
@@ -93,6 +93,7 @@
 
   /** Size **/
   --size-item-small: 16px;
+  --size-item-medium: 28px;
   --size-item-large: 32px;
 
   /** Spacing **/
diff --git a/toolkit/themes/shared/global-shared.css b/toolkit/themes/shared/global-shared.css
index ad573e4530cc..3d7ea499c54f 100644
--- a/toolkit/themes/shared/global-shared.css
+++ b/toolkit/themes/shared/global-shared.css
@@ -225,12 +225,19 @@ button.text-link .button-text {
   color: var(--button-color, inherit);
   background-color: var(--button-bgcolor, color-mix(in srgb, currentColor 13%, transparent));
   padding: .45em 1em;
-  min-height: 32px;
+  min-height: var(--size-item-large);
   font-weight: 600;
   min-width: 0;
   margin-inline: 8px 0;
   margin-bottom: 0;
 
+  &.small-button {
+    margin: 0;
+    min-height: var(--size-item-medium);
+    padding: .6em 1em;
+    font-size: var(--font-size-small);
+  }
+
   &[disabled] {
     opacity: 0.4;
   }
@@ -247,7 +254,8 @@ button.text-link .button-text {
     &:hover:active {
       background-color: var(--button-active-bgcolor, color-mix(in srgb, currentColor 30%, transparent));
     }
-    &[default] {
+    &[default],
+    &.primary {
       color: var(--button-primary-color);
       background-color: var(--button-primary-bgcolor);