Error removing child div second time

Question

I am using prototype 1.6.1 to insert and remove a div as a first child of another div. So I have parent div 'A'

and I do

$('A').insert(divB,'top');

then later

$('B').remove();

This works fine, but if I create the same div a second time and then try to remove it again I get an error that $('B').parentNode is null. But if I look at $('A').innerHTML it does show the child div 'B' inserted. I am seeing this only on firefox (on IE8 works fine).

Any hints of why this behavior?