Refactor ZenStartup.mjs to improve browser layout initialization and fix smooth scroll issue

src/browser/base/content/zen-styles/zen-tabs/vertical-tabs.css

@@ -50,6 +50,12 @@
           width: 3px !important;
         }
       }
+
+      @media (-moz-bool-pref: 'zen.tabs.dim-pending') { 
+        &[pending='true'] .tab-icon-image {
+          opacity: 0.5;
+        }
+      }
     }
   }