Firefox draws red border on required select box

Firefox 21 draws red borders around required select-boxes when they are bound to an angularjs-model.

<select ng-model="color" ng-options="c.text for c in colors" required>
   <option value="">-- choose color --</option>
</select>

Is there a way to let Firefox validate the input after selecting (or not selecting) an item?

→ A fiddle to demonstrate the problem