--- a/browser/components/extensions/ExtensionPopups.jsm
+++ b/browser/components/extensions/ExtensionPopups.jsm
@@ -363,17 +363,17 @@ class BasePopup {
     // of the extension content. Passing `null` should be treated the same as no argument,
     // which is why we can't use default parameters here.
     if (!background) {
       background = "#fff";
     }
     this.panel.style.setProperty("--arrowpanel-background", background);
     if (background == "#fff") {
       // Set a usable default color that work with the default background-color.
-      this.panel.style.setProperty("--arrowpanel-border-color", "hsla(210,4%,10%,.05)");
+      this.panel.style.setProperty("--arrowpanel-border-color", "hsla(210,4%,10%,.15)");
     }
     this.background = background;
   }
 }
 
 /**
  * A map of active popups for a given browser window.
  *