Bug 1610597 - Use legacy-stack for notification boxes. r=bgrins

Differential Revision: https://phabricator.services.mozilla.com/D60600

--HG--
extra : moz-landing-system : lando

toolkit/content/widgets/notificationbox.js

@@ -28,7 +28,7 @@
 
     get stack() {
       if (!this._stack) {
-        let stack = document.createXULElement("stack");
+        let stack = document.createXULElement("legacy-stack");
         stack._notificationBox = this;
         stack.className = "notificationbox-stack";
         stack.appendChild(document.createXULElement("spacer"));