You can put this hook code in the functions.php file of your current active WordPress theme or use Code Snippets plugin to insert this code. Delete all caches, then delete wpForo cache in Dashboard > Forums > Dashboard admin page.

add_filter('wpforo_post_list_args', function ($args){
   if(!empty($args) && !isset($_GET)){
      $args = '`is_first_post` DESC, `created` ASC, `postid` ASC';
   }
   return $args;
});