Firefox API to check if the tab is active

Question

I recall there was a functionality announced for some of recent Firefox releases that lets the developer to check if the tab is active or not with JavaScript but I can't find it now on the web. Could you please give me the link to the page with more information about it? I know I can check it more or less properly with focus/blur but want to try that FF feature out.

Answer 1

https://developer.mozilla.org/en/DOM/Using_the_Page_Visibility_API though note that this is about tabs being hidden not active . The difference matters for visible tabs in inactive windows.