Remove required checkbox border in Firefox

If a checkbox is required, Firefox will apply a red border to it, even if the form has not yet been submitted. How can I remove or prevent this border?

<input type='checkbox' required />

I tried applying the following styles, none of which worked. Now I'm out of ideas :/

input[type=checkbox]:invalid {
  -moz-appearance: none;
  appearance: none;
  border: none;
  outline: none;
}