Good, some usability problems Rated 3 out of 5 stars
Quite good extension, gives a functionality that firebug should already have implemented: autocompletion in the larger command line editor (as they did in the smaller one). But has many usability flaws.
@thomas: it's a problem with firebug 1.6, recently released. While the developer fixes it, you can download the xpi, unzip it, edit "install.rdf" and replace "1.6X.*" with "1.6.*" or "1.7X.*", then rezip and (re)install the extension. I think IT'S A GOOD IDEA TO DO TWO MORE CHANGES while you are at it: edit "chrome/skin/classic/style/overlay.css" and "chrome/content/style.js", replacing "background-color: -moz-menuhover !important;" with "background-color: highlight !important;", and "color: -moz-menuhovertext !important;" with nothing (which I recommend) or with "color: highlighttext !important;". That way you could solve one of the UI problems: the selected and hovered items have a background extremely clear.