Change post class if first post

firstpost_post_class.php

<?php 
    if ($wp_query->current_post == 0 && !is_paged()) :
        $first = ( $wp_query->current_post == 0 && !is_paged() ) ? 'firstpost' : ''; post_class($first);
    else :
        post_class();
    endif;
?>