Ho risolto cercando vari head in giro. funziona sia su ie che su firefox.

codice:
header('HTTP/1.1 200 OK');
header('Date: ' . date("D M j G:i:s T Y"));
header('Last-Modified: ' . date("D M j G:i:s T Y"));
header("Content-Type: application/force-download");
header("Content-Lenght: " . (string)(filesize("$path/$filename")));
header("Content-Transfer-Encoding: Binary");
header("Content-Disposition: attachment; filename=$filename");
@readfile("$path/$filename");
exit();