trello-card-counter-addon

Mozilla Firefox to add a card counter to trello list.

You should replace this extension with some custom CSS code. Create a chrome/userContent.css file in your Firefox profile and add this:

@-moz-document domain(trello.com) {
  .list-header-num-cards {
    display: block !important;
  }
}

This way the card counts automatically show up without any lag, no JS involved!