--- a/browser/themes/shared/customizableui/customizeMode.inc.css
+++ b/browser/themes/shared/customizableui/customizeMode.inc.css
@@ -457,17 +457,17 @@ toolbarpaletteitem[place=toolbar] > tool
 #customization-panelWrapper > .panel-arrowbox > .panel-arrow[side="top"] {
 %ifdef XP_MACOSX
   /* The OS X image is 2px narrower than the windows/linux one.
    * Add padding to compensate: */
   padding: 0 1px;
   /* specify width for hidpi image to fit correctly */
   width: 20px;
 %endif
-  list-style-image: url("chrome://global/skin/arrow/panelarrow-vertical.svg"));
+  list-style-image: url("chrome://global/skin/arrow/panelarrow-vertical.svg");
   -moz-context-properties: fill, stroke;
   fill: var(--arrowpanel-background);
   stroke: var(--arrowpanel-border-color);
   /* The arrow needs to point to the overflow button. The numbers involved
    * are:
    * overflow button width: (16px + 2 * 2px padding
    * + 2 * toolbarbutton-inner-padding)
    * hamburger button width: (16px + 2 * 5px padding