--- a/toolkit/themes/shared/notification-popup.inc.css
+++ b/toolkit/themes/shared/notification-popup.inc.css
@@ -2,10 +2,12 @@
   /* In order to display the action buttons near the edge of the arrow panel we
    * have to reset its default padding and specify the padding in the individual
    * "popupnotification" elements instead. To keep the rounded borders of the
    * panel, we also have to ensure the contents are clipped to the border box
    * by hiding the overflow, and we have to override the "display" property so
    * that the height of the contents is computed correctly in that case. */
   padding: 0;
   overflow: hidden;
-  display: block;
+  display: flex;
+  /* Make multiple popupnotifications stack vertically. */
+  flex-direction: column;
 }