1. Press and hold ctrl + shift and click on the thumbnail of the photo.
2. If this is the URL in the address bar while you are viewing a photo: http://www.facebook.com/#!/photo.php?fbid=XXXXXXXXX&set=at.XXXX.XXX.XXXX.XXXX.XXXX&theater lose the "&theater" at the very end.
There you go, good riddance bad photo viewer. Facebook, follow the golden rule: if it ain't broke...