Problem writing data to a form in another window

[Alan Hewat]

I open a new "myDoc" window with "myPage.html" containing "myForm" and
want to fill in the values of "myField" with some PHP variable
"$myVar". The following script:

1) works with MSIE-6, but only if I insert the alert (to give the
window time to load ? Is this really necessary ?)

2) with Mozilla-1.4 myPage.html opens but myField is not filled in.
Why not ?

myDoc=window.open('myPage.html','myDoc','width=620,height=600,left=10,top=10');
alert('Close this alert box to enter document details');
myDoc.focus();
myDoc.myForm.myField.value='$myVar';

Suggestions welcome. (I am not a javascript expert).