add firefox banner

_includes/header.html

@@ -27,4 +27,14 @@
     </p>
   </nav>
 
+  <span id='browser' class='notice hidden'>
+    This website is designed for <a target="_blank" rel="noopener noreferrer" href="https://firefox.com/">Firefox</a>, a web
+browser that respects your privacy.
+  </span>
+
+  <script>
+    if (window.chrome) {
+      document.getElementById('browser').className = 'notice';
+    }
+  </script>
 </header>

_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;
+}