Interesting little tool. Joe Hewitt's firebug is better but requires a plug in install to use.

https://addons.mozilla.org/en-US/firefox/addon/1843

firebug is an awesome tool but i think it's more for ajax/javascript debugging, though someone could prove me wrong

i should have put layout dissector in the title instead of the generic "ui tool"

It's really quite flexible. I find it helpful for debugging ASP even despite the travestied HTML that it outputs.

Try the Firebug 'inspect' button and then hovering/clicking in the source And CSS panels. Shows box model, computed CSS, allows live edits, and more.