Add featured thumbnail to admin post columns

functions.php

if ( function_exists( 'add_theme_support' ) ) {
    add_filter( 'manage_posts_columns' , 'frontlash_add_custom_columns' );
    add_action( 'manage_posts_custom_column' , 'frontlash_add_thumbnail_columns_data', 10, 2 );
    add_action( 'manage_posts_custom_column' , 'frontlash_add_modified_author_columns_data', 10, 2 );
}

function frontlash_add_custom_columns( $columns ) {
    $columns = array(
        'cb' => '<input type="checkbox" />',
        'title' => 'Title',
        'featured_thumb' => 'Thumbnail',
        'author' => 'Author',
        'the_modified_author'	=> 'Last modified',
        'categories' => 'Categories',
        'tags' => 'Tags',
        'comments' => '<span class="vers"><div title="Comments" class="comment-grey-bubble"></div></span>',
        'date' => 'Date'
    );
    return $columns;
}
function frontlash_add_thumbnail_columns_data( $column, $post_id ) {
    switch ( $column ) {
    case 'featured_thumb':
        echo '<a href="' . get_edit_post_link() . '">';
        echo the_post_thumbnail( 'admin-list-thumb' );
        echo '</a>';
        break;
    }
}
add_image_size( 'admin-list-thumb', 80, 80, false );
function frontlash_add_modified_author_columns_data($column, $post_id){
	switch ( $column ) {
    case 'the_modified_author':
        echo the_modified_author();
        echo '<br>';
        echo get_post_modified_time('F d, g:i a');
        break;
    }
}