FF keypress select oddity

Pressing enter only raises a keypress event on a select if the multiple attribute is not present

HTML

<body>
<select id="search" multiple>
<option>Test one</option>
<option>Test two</option>
<option>Test three</option>
</select>
    <div id="results"></div>

JavaScript

function processkeypress(e) {
    $('#results').append('Key: '+e.which+'<br />');
}
$(document).ready(function(){
    $('#search').focus();
    $('#search').keypress(processkeypress);
});