Posting by developer Honza Bamba: “Firefox Quantum – version 57 – introduced number of changes to the network requests scheduler.  One of them is using data of the Tracking Protection database to delay load of scripts from tracking domains when possible during the time a page is actively loading and rendering – I call it tailing. This has a positive effect on page load performance as we save some of the network bandwidth, I/O and CPU for loading and processing of images and scripts running on the site so the web page is complete and ready sooner. Tracking scripts are not disabled, we only delay their load for few seconds when we can. Requests are kept on hold only while there are site sub-resources still loading and only up to about 6 seconds. The delay is engaged only for scripts added dynamically or as async. Tracking images and XHRs are always delayed, as well as any request made by a tracking script. This is legal according all HTML specifications and it’s assumed that well built sites will not be affected regarding functionality…”