--- a/browser/themes/osx/devedition.css
+++ b/browser/themes/osx/devedition.css
@@ -9,17 +9,17 @@
   --search-button-image: url("chrome://browser/skin/devedition/search.svg#search-icon-mac-inverted");
 }
 :root[devtoolstheme="light"] {
   --search-button-image: url("chrome://browser/skin/devedition/search.svg#search-icon-mac");
 }
 
 /* Use only 1px separator between nav toolbox and page content */
 #navigator-toolbox::after {
-  linear-gradient(to top, var(--chrome-navigator-toolbox-separator-color), var(--chrome-navigator-toolbox-separator-color) 1px, transparent 1px);
+  background: linear-gradient(to top, var(--chrome-navigator-toolbox-separator-color), var(--chrome-navigator-toolbox-separator-color) 1px, transparent 1px);
 }
 
 /* Include extra space on left/right for dragging since there is no space above
    the tabs */
 #main-window[tabsintitlebar] #TabsToolbar {
   padding-left: 50px;
   padding-right: 50px;
   margin-bottom: 0; /* Don't overlap the inner highlight at the top of the nav-bar */