The pop-up window should be opened next to the main window on a second monitor. I tested it at the beginning of April this year and it worked.

Can this be re-enabled via about:config?

var windowObjectReference;

function openRequestedPopup() {
  windowObjectReference = window.open(
    "popup.php",
    "DescriptiveWindowName",
    "left=1920,top=0,width=1920,height=1080,popup"
  );
}

setTimeout(openRequestedPopup,1000);