I found it this yesterday, and it’s already been helpful in getting UI stuff done. While it’s not in the realm of Firebug (as a tool no web developer should be without) I’m impressed.
UPDATE: The blog entry introducing the tool seems to be down at the moment. But here’s the bookmarklet. You can use this to get a layout grid, arbitrary pixel measurements, and other design data visually right in Firefox. Just drag the bookmarklet to your bookmark menu and click on it in any page you want to measure/explore.