Yeah I had the same problem using the stable channel. I switched to the debug channel & it worked perfectly, opening right up. So use this:
<script type="text/javascript" src="https://getfirebug.com/firebug-lite-debug.js"></script>
Also, I find it useful to throw a quick 300px-high div in there at the bottom of my page to make room for Firebug Lite, like this:
<div style="display:block; height:300px; background:blue;"></div>
Just don't forget to remove it with the FBL script tag!