<select>
    <option>Jump to a letter: </option>
    <option>A</option>
    <option>B</option>
    ...
</select>

jQuery('.glossary-mobile-nav select').change(function () {
    var id = jQuery(this).val();
    var target = jQuery('.glossary-list div[id="'+id+'"]');

    jQuery('html, body').animate({
        scrollTop: target.offset().top
    }, 1000);
});