chkControl.form element is null in mozilla why ?

Question

I am using ajax to populate the multilevel checkboxes dynamically depending on the users selections. Like choosing one or more options from first checkbox group will update new checkbox list and so on. I am adding the checkboxes to div. All the checkboxes are arranged using the table structure inside the form element.

Now to access checkbox as a group i take the checkbox controls on checkbox click using function selectElement(this) using which i get the checkbox and using which i retrive the Form control. However in Mozilla the chkControl.form is coming null and after trying to find solution i failed to find why ? can nesting form element wrongly in table structure cause such an issue ?

function selectElement(chkControl)
{
   var frm = chkControl.form; //this in mozilla is returning null
   //frm processing code
}