Hi @Natim,

This is not really an issue with your code, just an alternative solution to your extension.

Thanks to your open-source extension code, I discovered that Trello was computing this information but hiding it by default!

We can replace your 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!

Cheers

I noticed that after some minutes that the page is open the card count disappear.

I don't know how much time, but can be something like one hour, I keep always a tab open on trello and sometimes when i come back on that tab the count is no more there.

Trying with Firefox 38 Mac Yosemite 10.10.3. Installed plugin but count doesn't appear in Trello. Is there anything specific to be done for the same?