Do you want to wrap comments in a collapsible fieldset in Drupal 7?

Add the following function to your template.php of your theme:

/**
 * Wraps comments in a collapsible fieldset.
 */
function YOUR_THEME_preprocess_comment_wrapper(&$variables) {
  // Provide contextual information.
  $variables['node'] = $variables['content']['#node'];
  $variables['display_mode'] = variable_get('comment_default_mode_' . $variables['node']->type, COMMENT_MODE_THREADED);
  // The comment form is optional and may not exist.
  $variables['content'] += array('comment_form' => array());
  $commentslist = $variables['content']['comments'];
  $variables['content']['comments'] = array(
    '#type' => 'fieldset',
    '#title' => format_plural($variables['node']->comment_count, '1 comment', '@count comments'),
    '#weight' => 0,
    //'#collapsible' => TRUE, // does not work and thus is not necessary
    //'#collapsed' => TRUE, // does not work and thus is not necessary
    '#pre_render' => array(),
    '#attributes' => array(),
  );
  // Make it manually collapsible as we cannot use #collapsible without the
  // FAPI element processor.
  $variables['content']['comments']['#attached']['library'][] = array('system', 'drupal.collapse');
  $variables['content']['comments']['#attributes']['class'][] = 'collapsible';
  $variables['content']['comments']['#attributes']['class'][] = 'collapsed';
  $variables['content']['comments']['commentslist'] = $commentslist;
}