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

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.