How to set custom http headers when changing iframe src?

Question

Is there a way to add a custom http header into the request done by an

Edit 2:

The solution above appears to no longer be working in Firefox (50.1.0) for some unknown reason. Using the solution in this answer I've now changed to code to the example below, which also seems to be more robust:

$.ajax({
    type: "GET", 
    url: "https://yourdomain.com/gethtml",
    beforeSend: function(xhr) {
        xhr.setRequestHeader("yourheader", "value");
    },
    success: function(data) {
        var iframe = document.getElementById('myiframe');
        iframe.contentWindow.contents = data;
        iframe.src = 'javascript:window["contents"]';
    }
});