Try changing function fontEditor()
to function fontEditor(fontName)
.
function fontEditor(fontName) {
document.execCommand("fontName", false, fontName);
...
}
And:
<select onchange="fontEditor(this[this.selectedIndex].value)">
<option value="Arial">Arial</option>
<option value="Calibri">Calibri</option>
<option value="Comic Sans MS">Comic Sans MS</option>
</select>