Here is something in plain HTML with a bit of JavaScript. Just tested in FF and works fine.
(If I understood you correctly
)
code:
<html>
<body>
<form name="form1" target="ifr">
<input type="text" name="surl" size="40" /><br />
<input type="button" value="Open" onClick="document.getElementById('ifr').src=document.form1.surl.value" /></form>
<iframe id="ifr" width="100%" height="100%" frameborder="1"></iframe>
</body>
</html>