--- a/toolkit/themes/linux/global/notification.css
+++ b/toolkit/themes/linux/global/notification.css
@@ -6,8 +6,12 @@
 
 %include ../../shared/notification.inc.css
 %include ../../shared/popupnotification.inc.css
 
 .popup-notification-button:-moz-focusring {
   outline: 1px -moz-dialogtext dotted;
   outline-offset: -5px;
 }
+
+.popup-notification-button[anonid="secondarybutton"]:not([hidden="true"])  ~ .popup-notification-button[default]:not([highlight="true"]) {
+  border-inline-start: 1px solid var(--panel-separator-color);
+}

--- a/toolkit/themes/osx/global/notification.css
+++ b/toolkit/themes/osx/global/notification.css
@@ -11,8 +11,12 @@
 .popup-notification-button:-moz-focusring {
   outline: 2px -moz-mac-focusring solid;
   outline-offset: -2px;
 }
 
 .popup-notification-warning {
   color: #aa1b08;
 }
+
+.popup-notification-button[anonid="secondarybutton"]:not([hidden="true"])  ~ .popup-notification-button[default]:not([highlight="true"]) {
+  border-inline-start: 1px solid var(--panel-separator-color);
+}

--- a/toolkit/themes/windows/global/notification.css
+++ b/toolkit/themes/windows/global/notification.css
@@ -19,8 +19,12 @@
   }
 }
 
 /* Swap the default and secondary action, because Windows
  * platform conventions put the default action on the left. */
 .popup-notification-button[default] {
   order: -1;
 }
+
+.popup-notification-button[anonid="secondarybutton"]:not([hidden="true"])  ~ .popup-notification-button[default]:not([highlight="true"]) {
+  border-inline-end: 1px solid var(--panel-separator-color);
+}