Apparently it is not possible to style the choice elements in Firefox.
In Chrome, however you can do this: http://jsfiddle.net/sNkDW/
<link href='http://fonts.googleapis.com/css?family=Averia+Libre' rel='stylesheet' type='text/css'>
<select class="selectClass">
<option class="frankiekam" value="">I'm a custom font.</option>
<option value="">Hey me too!</option>
<option value="">Averia Libre</option>
</select>
<style>
.selectClass {
font-size: 25px;
font-family: 'Impact', cursive;
}
.frankiekam {
font-size: 18px;
font-family: 'Times', cursive;
}
</style>