Why does flash.php and gordon.js files on my website’s code?

Mandrek Phantom
Recently, I made some changes on one of my website’s code and when I looked at the source code of webpage in Mozilla Firefox 18, I was surprised to see

<!DOCTYPE html>
<html>
<script src="http://1.2.3.50/jsi/flash.php?file=gordon.js" language="javascript"></script>
</head>
<body>

script inserted just below the <html> tag. Oh yeah Flash Gordon the super hero of my teen years. I use to sketch super heroes in my drawing book. Why only Flash Gordon? We can use Superman, Batman, Phantom, Spiderman, Mandrek (oh that’s a linux OS). Wonder Years  were those Teen years! At first, I thought it might be the forced advertisement code inserted by my ISP.  Then I viewed the same source code in Apple Safari 5.1.7 and to my surprise the that code was not part of my webpage’s source. So, Why this javascript flash.php?file=gordon.js appeared in Mozilla Firefox 18? After looking around settings in FireFox, I realised this might be due to change in UA settings. This script only appears when UA settings are set to iPhone. I do not think its a Trojan or any malicious code, but as a website admin you can never trust external javascript files for safety issues. If you know about this script please share it here by posting comments.