[jQuery] attr('form') not work in firefox

[[EMAIL PROTECTED]]

This is my html:
<form id="myForm" action="">
<input type="text" id="a" />
</form>
and javascript:
<script type="text/javascript>
$(document).ready(function() {
    var form = $('#a').attr('form');
    alert(form.attr('action'));
});
</script>
it works in IE 7.0, but in FF form is 'undefined', I can get the form
using
var form = $('#a').get(0).form;

so why attr('form') does not work in firefox?