--- a/browser/themes/shared/customizableui/panelUI.inc.css
+++ b/browser/themes/shared/customizableui/panelUI.inc.css
@@ -1730,16 +1730,23 @@ toolbarpaletteitem[place=panel] > .toolb
   overflow-y: auto;
   overflow-x: hidden;
 }
 
 .widget-overflow-list {
   width: @wideMenuPanelWidth@;
 }
 
+/* In customize mode, the overflow list is constrained by its container,
+ * so we set width: auto to avoid the scrollbar not fitting.
+ */
+#customization-panelHolder > .widget-overflow-list {
+  width: auto;
+}
+
 toolbaritem[overflowedItem=true],
 .widget-overflow-list .toolbarbutton-1 {
   width: 100%;
   max-width: @wideMenuPanelWidth@;
   background-repeat: no-repeat;
   background-position: 0 center;
 }