--- a/addon-sdk/source/lib/sdk/panel/utils.js
+++ b/addon-sdk/source/lib/sdk/panel/utils.js
@@ -250,17 +250,17 @@ function onPanelClick(event) {
 }
 
 function setupPanelFrame(frame) {
   frame.setAttribute("flex", 1);
   frame.setAttribute("transparent", "transparent");
   frame.setAttribute("autocompleteenabled", true);
   frame.setAttribute("tooltip", "aHTMLTooltip");
   if (platform === "darwin") {
-    frame.style.borderRadius = "6px";
+    frame.style.borderRadius = "var(--arrowpanel-border-radius, 3.5px)";
     frame.style.padding = "1px";
   }
 }
 
 function make(document, options) {
   document = document || getMostRecentBrowserWindow().document;
   let panel = document.createElementNS(XUL_NS, "panel");
   panel.setAttribute("type", "arrow");