Why form input[type="checkbox"] is considered as readonly by Firefox but it shouldn't?

Firefox considers my form input[type="checkbox"] as readonly but it shouldn't as the field is editable. It works on every other browser including chrome or even IE11.

The code looks like below: HTML:

<input name="upgradeSmu" id="id_fd5049f8-8549-4b0e-8f81-afbfe927d562" type="checkbox">

CSS:

form input:-moz-read-only, form input:-moz-read-only:hover, form input:-moz-read-only:focus {
  border-color: #dddddd !important;
  background: transparent;
  cursor: not-allowed; }

Firefox for some reason gets this CSS as I see it in computed section:

Chrome shows me following:

Firefox version: 65.0.2.