I don't know your source, but code like this will pop under in those browsers and not pop up:
var url = "yourURL.html";
window.open(url, "s", "width= 640, height= 480, left=0, top=0, resizable=yes, toolbar=no, location=no, directories=no, status=no, menubar=no, scrollbars=yes, resizable=no, copyhistory=no").blur();
window.focus();
(I know that this at least works in Chrome where regular popunder scripts pop up instead.)

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