Bug 1642323 - Make alert.js obey prefers-reduced-motion. r=Gijs

Differential Revision: https://phabricator.services.mozilla.com/D78032

browser/base/content/test/alerts/head.js

@@ -1,3 +1,9 @@
+// Platforms may default to reducing motion. We override this to ensure the
+// alert slide animation is enabled in tests.
+SpecialPowers.pushPrefEnv({
+  set: [["ui.prefersReducedMotion", 0]],
+});
+
 async function addNotificationPermission(originString) {
   return SpecialPowers.pushPermissions([
     {

toolkit/components/alerts/resources/content/alert.js

@@ -211,9 +211,7 @@ function onAlertLoad() {
 
   // If the require interaction flag is set, prevent auto-closing the notification.
   if (!gRequireInteraction) {
-    if (
-      !Services.prefs.getBoolPref("toolkit.cosmeticAnimations.enabled", true)
-    ) {
+    if (window.matchMedia("(prefers-reduced-motion: reduce)").matches) {
       setTimeout(function() {
         window.close();
       }, ALERT_DURATION_IMMEDIATE);