FF: Cursor Position is off when absolute pseduo-content

HTML

<article contenteditable></article>

CSS

body {padding:20px;}

[contenteditable] {
    position:relative;
    border:1px solid #CCC;
    box-shadow:none;
    min-height:20px;
}

[contenteditable]:after {
    content:'focus me (the cursor is off)';
    display:block;
    color:#ccc; font-style:italic;
    position:absolute;
    right:5px;
}

[contenteditable]:after {
    display: none;
}