Button Firefox bug

HTML

<button><span></span></button>

<button><div><span></span></div></button>

CSS

button {
    -moz-appearance: none; /* Just in case */
    -webkit-appearance: none; /* Just in case */
    position: relative;
    height: 100px;
    width: 100px;
    display: block;
    background: red;
    border: 0;
    padding: 0;
    margin: 5px;
}

div {
    background: yellow;
    height: 100px;
    width: 100px;
}

span {
    position: absolute;
    top: 10px;
    right: 10px;
    bottom: 10px;
    left: 10px;
    background: green;
}