Try this something like that:
<a href="#" id="nav-trigger">
<span style="font-family: Arial;">MENU</span> m
</a>
The problem you are having is that the menüicon is not an inline- or background-image but a font character. The a-tag uses a font in wich the m stands for this menuicon. So putting another element inside the a-tag and giving it another font-family should do the trick.