[jQuery] is(":empty") works in IE but not firefox

[Subtle]

I'm calling a server side procedure  using $.ajax that returns a table
of data and for one of the columns it can include blank tds.  I have
code to check for the blank tds.
theData.each(
      function()
      {
            var theFirstColumn = $(this).find('td:eq(0)');
            if(!theFirstColumn.is(":empty"))
            {
                  // do something
            }
});
In IE 7/8 when the markup of the cell looks like this: "<td></td>" the
"is empty" check returns true.  In Firefox 3.5, it returns false.

Any ideas?

Thanks.