fix Syle of dropdown element in FF29 an later

- remove custom dropdown arrow in firefox because it's too complicated to hide the native one (in other browsers its one line of css)
- make the dropdown arrow click-through so that one can click it to show the dropdown

app/assets/stylesheets/zammad.css.scss

@@ -665,6 +665,24 @@ textarea,
     right: 12px;
     top: 50%;
     margin-top: -3px;
+    @extend .u-unclickable;
+  }
+
+  /* 
+
+    Firefox only hack
+    -----------------
+
+    Firefox doesn't allow us to hide the dropdown arrow
+    but we want to replace it with our own icon.
+    So we have to hide our own icon in Firefox.
+
+  */
+
+  @-moz-document url-prefix() {
+    .form-control + .select-arrow {
+      display: none;
+    }
   }
 
   select::-ms-expand {
@@ -3687,6 +3705,7 @@ footer {
 .type-tabs .tab {
   line-height: 42px;
   padding: 0 12px 0 12px;
+  list-style: none;
 }
 
 .type-tabs .icon {