Autofilling not working on own website

[Nighthawk]

Hello,

The firefox extension (v. 5.3.4) does not work on a website i created.
The login form has the following html code:
 

<div class="form__field">
  <label><!-- react-text: 146 -->Username<!-- /react-text --><!-- react-text: 147 -->:<!-- /react-text -->
    <input step="" name="username" value="" class="" type="text">
    <p class="help-text">Usernames are not e-mail addresses.</p>
  </label>
</div>
<div class="form__field">
    <label><!-- react-text: 152 -->Password<!-- /react-text --><!-- react-text: 153 -->:<!-- /react-text -->
      <input step="" name="password" value="" class="" type="password">
  </label>
</div>
<div class="login__sumode">
  <div class="form__field">
    <label><!-- react-text: 158 -->Target user<!-- /react-text --><!-- react-text: 159 -->:<!-- /react-text -->
      <input step="" name="targetuser" value="" type="text"></label>
  </div>
</div>
<div class="form__actions">
  <button type="submit"><span>Login</span></button>
</div>

The target user is hidden with CSS.
 

Any idea?

Thanks,
Nighthawk