[Javascript] Firefox - write to form boxes

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http://www.mapability.com/firefox/frameset.htm

Hi - I am having some problems with the FireFox browser, when I ask it to
write some values into some form boxes.

Above is the link to a frameset with 2 frames:
<frameset rows="100,280" framespacing="0" border="true" border="false"
frameborder="0">
<frame name="nav" target="map" src="navbar.html" scrolling="no" noResize
border="false" frameBorder="0" marginHeight="0" marginWidth="0">
<frame name="map" noResize border="false" frameBorder="0" marginHeight="0"
marginWidth="0" src="map.html">


In the top frame is a function:
function write_box(){
parent.map.F7.box1.value = 'Test1';
parent.map.F7.box2.value = 'Test2';
parent.map.F7.box3.value = 'Test3';
}


In the bottom frame is a form with some boxes, and a button:
<form name="F7">
Testing FireFox<br>
Box1: <input onfocus="blur()" size="10" name=box1> Box2: <input class="fbox"
onfocus="blur()" size="10" name=box2> Box3: <input class="fbox"
onfocus="blur()" size="10" name=box3> &nbsp; <input type="button"
value="Write some values.." onclick="parent.nav.write_box()" /></div></form>


Viewing the above link in Internet Explorer, and clicking on the button,
writes the values into the form boxes, but with FireFox it doesn't. Why?

Tim in Ireland.