I have used an iframe which looks like this:
The frames collection returns window objects (or the equivalent of). You want to target the document object; try doing:
window.frames['iframeId'].document.location.href = ....
This works in IE, FF, Safari, and so on, so no need for the messy browser detection too.
nb. IIRC the frames collection references name in IE, id in other browsers, so you need both name and id attribute on the - but you already have that, so no worries!