Hmmm… I haven’t tried Headerhacker so I downloaded it just now and explored it a little. It would seem that when you begin typing “X-Forwarded-For” in the Header field, it tries to autocomplete and offers a dropdown list from which you can select X-Forwarded-For. Then you change the Append/Replace to Append and add the IP address (126.96.36.199) in the String field.
After I did all that, I watched it with Wireshark and it appears to be appending the correct header. However, the site still won’t display the video. I know for a fact, though, that ModHeader works. Why not try that extension instead?
After you download and install it, click the icon in the toolbar, enter X-Forwarded-For in the first field and 188.8.131.52 in the second field and add it. That’s all you need to do.