fx contenteditable bug

HTML

<div id="c">
    <p>
        Select some of this text then press one of the buttons.     
    </p>
    <p>
    The `styleWithCss=true` button should generate a span with font-size styles.
    </p>
    <p>
    While the `styleWidthCss=false` button should generate a font tag around the selected text.
    </p>
    <p>
    Instead, on Firefox 29, both buttons generate a font tag. 
    </p>
    <p>
    Seems to work correctly on Chrome 37.
    </p>
</div>

<button id="b1">
    styleWithCss=true
</button>

<button id="b2">
    styleWithCss=false
</button>

JavaScript

var $b1 = document.getElementById('b1');
var $b2 = document.getElementById('b2');

$b1.addEventListener('click', function() {
    document.execCommand('styleWithCSS', false, true);
    document.execCommand('fontSize', false, 7);
});

$b2.addEventListener('click', function() {
    document.execCommand('styleWithCSS', false, false);
    document.execCommand('fontSize', false, 7);
});