--- a/browser/themes/shared/customizableui/panelUI.inc.css
+++ b/browser/themes/shared/customizableui/panelUI.inc.css
@@ -87,24 +87,24 @@
   /* These values are adjusted for the padding and height of the panel. */
   from { margin-top: -.5em; } to { margin-top: calc(64px - .5em); }
 }
 
 @keyframes whimsyRotate {
   to { transform: perspective(5000px) rotateY(360deg); }
 }
 
-#PanelUI-button {
+:root:not([uidensity=compact]) #PanelUI-button {
   margin-inline-start: 3px;
   border-inline-start: 1px solid;
   border-image: linear-gradient(transparent 4px, rgba(0,0,0,.1) 4px, rgba(0,0,0,.1) calc(100% - 4px), transparent calc(100% - 4px));
   border-image-slice: 1;
 }
 
-#nav-bar[brighttext] > #PanelUI-button {
+:root:not([uidensity=compact]) #nav-bar[brighttext] > #PanelUI-button {
   border-image-source: linear-gradient(transparent 4px, rgba(100%,100%,100%,.2) 4px, rgba(100%,100%,100%,.2) calc(100% - 4px), transparent calc(100% - 4px));
 }
 
 #PanelUI-menu-button[badge-status] > .toolbarbutton-badge-stack > .toolbarbutton-badge {
   display: -moz-box;
   height: 10px;
   width: 10px;
   background-size: contain;