add firefox banner

_sass/_base.scss

@@ -13,6 +13,10 @@ dl, dd, ol, ul, figure {
   text-align: center;
 }
 
+.hidden {
+  display: none !important;
+}
+
 
 /**
  * Basic styling

_sass/_layout.scss

@@ -271,3 +271,7 @@ hr {
     margin: 0;
   }
 }
+
+#browser {
+  display: inline-block;
+}