Javascript : parent.formMain.name.value -- Not working in Firefox [RESOLVED]

[TomGibbons]

In the parent page I have this:

<form name="formMain">
      <input type="text" name="name" />
</form>

<iframe src="iframe.htm" width="300" height="300"></iframe>

iframe.htm:

<input type="button" onclick="parent.formMain.name.value='new value'" value="Click me" />

Works fine in IE, which most (if not all) of the clients will use anyway, but I use Firefox to develop and I wondered why it's not working