An extension to FireFox, Web Developer, has many features that would compliment the HTML tool if they could be incorporated. A link to the extension page is provided below.
http://www.chrispederick.com/work/firefox/webdeveloper/features/
Color Zilla may be a nice addition. The only problems is that it confines it’s self to the browser. A color picker that worked any place on the screen regardless would be helpful.
http://www.iosart.com/firefox/colorzilla/
The DOM inspector is invaluable for tracking down problems.
http://packages.debian.org/testing/web/mozilla-firefox-dom-inspector