<?php
$u = $_GET['url'];
echo $u;
$u = rawurlencode($u);
if (isset($u)&&trim($u!="")){
   exec('DISPLAY=:0 firefox "http://localhost/openInPopup.php?url='.$u.'"');
} 
?>
<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1">
<form action=index.php>
   <input name=url autofocus>
   <button>go</button>
</form>
<form action=index.php>
   <input type=hidden name=url value=about:blank>
   <button>stop</stop>
</form>

<input id=url value="<?=rawurldecode($_GET['url'])?>" />
<script>
  window.open(document.getElementById('url').value,"playToPopup",'height=500,width=1000');
</script>

firefox javascript:window.open( ... )