diff --git a/browser/base/content/urlbarBindings.xml b/browser/base/content/urlbarBindings.xml
index d3407aa58100..444c48057faf 100644
--- a/browser/base/content/urlbarBindings.xml
+++ b/browser/base/content/urlbarBindings.xml
@@ -2886,6 +2886,12 @@ file, You can obtain one at http://mozilla.org/MPL/2.0/.
           } else if (button2.default) {
             this._secondaryButton.setAttribute("default", "true");
           }
+
+          if (this._primaryButton.hidden) {
+            this._secondaryButton.setAttribute("alone", "true");
+          } else if (this._secondaryButton.hidden) {
+            this._primaryButton.setAttribute("alone", "true");
+          }
         ]]></body>
       </method>
       <method name="_setupDescription">
diff --git a/toolkit/themes/shared/popupnotification.inc.css b/toolkit/themes/shared/popupnotification.inc.css
index 0589a9a705d6..d48c18b1a47e 100644
--- a/toolkit/themes/shared/popupnotification.inc.css
+++ b/toolkit/themes/shared/popupnotification.inc.css
@@ -78,7 +78,7 @@
   color: graytext;
 }
 
-.popup-notification-button[default] {
+.popup-notification-button[default]:not([alone]) {
   flex: 0 50%;
 }