What's missing from these image headers?

I'm working on a site which is showing images that are parsed

Example Page: http://www.findmyrental.co.nz/adlistings/view/117
Example Image: http://members.nzpif.org.nz/adimages/thumb/151

If I view the page in IE, Chrome, Safari and Opera its fine.
If my husband views it in Firefox its fine

If I view it in Firefox the images are all hidden and have dodgy classes added to the html by "webkit"

At one point I was getting errors shown in Chrome saying that the headers were wrong but it could see that they were actually images.

My code is pretty straightforward

    $output = readfile($filetofetch);
        if (isset($dateModified) && !empty($dateModified)) {
        header("Date: " . date("D, j M Y G:i:s ", $dateModified) . 'GMT');
    }
    header("Content-Type: ".$mime);
    header("Expires: " . gmdate("D, j M Y H:i:s", time() + DAY) . " GMT");
    header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
    header("Cache-Control: private",false); // required for certain browsers
    header("Pragma: public");
        // HTTP/1.0    header("Content-Transfer-Encoding: binary");
     echo $output;
    exit;

PHP:

So, what am I missing?

 

Well that's embarrassing!

I've stopped using FF as my main browser and totally forgot about the adblocker