--- a/toolkit/themes/shared/popupnotification.inc.css
+++ b/toolkit/themes/shared/popupnotification.inc.css
@@ -31,17 +31,16 @@
 }
 
 .popup-notification-learnmore-link {
   margin-top: .5em !important;
 }
 
 .popup-notification-button-container {
   background-color: var(--arrowpanel-dimmed);
-  border-top: 1px solid var(--panel-separator-color);
   display: flex;
 }
 
 .popup-notification-button-container > toolbarseparator {
   -moz-appearance: none;
   border: 0;
   border-left: 1px solid var(--panel-separator-color);
   margin: 7px 0 7px;
@@ -55,34 +54,38 @@
 .popup-notification-button {
   flex: 1;
   -moz-appearance: none;
   background-color: transparent;
   color: inherit;
   margin: 0;
   padding: 0;
   min-width: 0;
-  min-height: 40px;
+  min-height: 41px;
   border: none;
+  border-top: 1px solid var(--panel-separator-color);
 }
 
 .popup-notification-button:hover:not([disabled]) {
   outline: 1px solid var(--arrowpanel-dimmed);
   background-color: var(--arrowpanel-dimmed);
 }
 
 .popup-notification-button:hover:active:not([disabled]) {
   outline: 1px solid var(--arrowpanel-dimmed-further);
   background-color: var(--arrowpanel-dimmed-further);
   box-shadow: 0 1px 0 hsla(210,4%,10%,.05) inset;
 }
 
 .popup-notification-button[disabled] {
   background-color: var(--arrowpanel-dimmed-further);
   color: graytext;
+  /* the !important is to override the button[disabled] settings in
+   * windows/linux button.css */
+  -moz-border-top-colors: var(--panel-separator-color) !important;
 }
 
 .popup-notification-button[default] {
   flex: 0 50%;
 }
 
 .popup-notification-button[default]:not([disabled]) {
   background-color: #0996f8;