Add babel polyfill and tweak some styles for firefox

styles/base.css

@@ -40,3 +40,7 @@ body {
     color: blue;
   }
 }
+
+#root {
+  min-height: 100vh;
+}

styles/components/branding.css

@@ -4,6 +4,8 @@
   align-items: center;
   justify-content: space-between;
   min-height: 60px;
+  transition: opacity 450ms cubic-bezier(0.24, 1.12, 0.71, 0.98);
+  opacity: 1;
 
   &__backlink {
     flex: 0 0 auto;
@@ -19,4 +21,8 @@
   &__content {
     flex: 0 1 auto;
   }
+
+  &.is-hidden {
+    opacity: 0;
+  }
 }

styles/components/footer.css

@@ -0,0 +1,9 @@
+.footer {
+  border-top: 1px solid var(--color-gray-normal);
+  transition: opacity 450ms cubic-bezier(0.24, 1.12, 0.71, 0.98);
+  opacity: 1;
+
+  &.is-hidden {
+    opacity: 0;
+  }
+}

styles/components/page.css

@@ -5,6 +5,7 @@
   padding: 0 10px;
   display: flex;
   flex-direction: column;
+  min-height: 100vh;
 
   &__header {
     flex: 0 0 auto;
@@ -18,7 +19,6 @@
   }
 
   &__footer {
-    border-top: 1px solid var(--color-gray-normal);
     flex: 0 0 auto;
   }
 }