title: Angular 2 validation and checkbox 'required' attribute does not work on Firefox
body: <p>I have a form in my Angular 2 app that has a single radio button question (with 3 answer options) in it that is required.  It works great in Chrome but when I select an answer to the radio button in Firefox, then press the submit button, a popup over the browser <code>back</code> button says "please select one of these options" and won't let me submit the form.</p>
  
  <p>Why is this happening and how can I fix it?</p>
  
  <p>Some of the html code:</p>
  
  <pre><code>&lt;form (ngSubmit)="onNext(f)" #f="ngForm"&gt; &lt;!-- section form --&gt;
  
      &lt;div class="question-div" *ngFor="let question of section?.questions"&gt;
          &lt;h3 class="question-text"&gt;{{ question?.name }}&lt;/h3&gt;
  
          &lt;!-- for multichoice questions --&gt;
          &lt;div class="multi-choice-question-div radio-btn-div question_div" 
              *ngIf="question?.type === 'multi-choice' &amp;&amp; !question?.isYesOrNo"&gt;
              &lt;div *ngFor="let answer of question?.answerDetails"&gt;
                  &lt;input
                      type="radio" 
                      class="display-none" 
                      id="{{ answer?.answerId }}"
                      [(ngModel)]="ngModelObj['question_' + question.questionId]"
                      (ngModelChange)="onAnswerChanged()"
                      name="answerForQustion{{ question?.questionId }}"
                      [value]="answer"
                      required&gt;
                  &lt;label class="col-md-12 col-sm-12 multi-choice-label" for="{{ answer?.answerId }}"&gt;
                      &lt;p class="q-text"&gt;{{ answer?.value }}&lt;/p&gt;
                  &lt;/label&gt;
              &lt;/div&gt;
          &lt;/div&gt;
          &lt;!-- end multichoice question --&gt;
  
          ....
  
      &lt;button
          type="button"
          class="btn btn-next"
          (click)="onNext(f)"
          *ngIf="currentSectionIndex === sectionsArr.length - 1"
          [disabled]="!checkFormValidation(f)"&gt;
              NEXT
      &lt;/button&gt;
  
  &lt;/form&gt;
  </code></pre>