Firefox 57 includes a secret feature that will delay the loading of tracking scripts from known tracking domains such as advertising, analytics, and social networks. Firefox will delay scripts only when they are added dynamically to a page or via an async call. Tracking scripts are delayed for a maximum of six seconds, after which they are loaded regardless if the underlying web page has finished loading or not. Users can edit the first two parameters (network.http.tailing.delay-quantum-after-domcontentloaded) to change the minimum delay values.
Source: https://www.bleepingcomputer.com/news/software/firefox-will-now-delay-the-loading-of-tracking-scripts/