The problem occurs because the first node returned for IE is the comment node. Here is my attempt at fixing it. (checked in IE6/7 and FF2/3).

"first-child": function(a){
  var isFirst = false;
  $.each(a.parentNode.getElementsByTagName("*"), function() {
    // check if the first non-comment node is same as the node.
    return (this.tagName === '!') || ((isFirst = (this == a)) && false);
  });
  return isFirst;
}

In depth test case.

I've done a much more in-depth test case and determined that this is the only one of the similar selectors that exhibits this bug. It appears that .getElementsByTagName("*") returns comment nodes in IE. I'm working on a patch.

Patch

To fix this we're going to have to go slower and longer. A more clever patch might be better than the one I've just posted for review.