Load value from master window into pop-up window (javascript)

I would like to populate a field in the pop-up window with a field value located in the master window.

Basically my code works with IE but not with FireFox, any ideas:

document.forms[0].RegNo.value = window.opener.document.getElementById(‘RegNo’).innerHTM;

Subject: Load value from master window into pop-up window (javascript)

Try this

document.forms[0].RegNo.value = window.opener.document.forms[0].RegNo.value;