You can't just save it as an HTML file on your dev machine and open it in firefox: it needs to be opened from a server which supports PHP.
If you don't then the php code can;t be executed so you will get a blank display.
Try it:
<!DOCTYPE html>
<html lang="en" dir="ltr">
<head>
<meta charset="utf-8">
<title></title>
</head>
<body>
This machine <?php echo "supports php"; ?>
</body>
</html>
Will print "this machine" but nothing else.