Hi The "DOM Inspector" addon should be added to your list. It is available at: https://addons.mozilla.org/en-US/firefox/addon/dom-inspector-6622/
According to that page it uses a Custom Licence, but inside that licence it states that it is the tri-licence "MPL 1.1/GPL 2.0/LGPL 2.1". This addon aids in developing, or at least learning from, XUL-based addons by allowing a user to inspect the UI widgets and viewing the underlying XML source. Thanks Morne Alberts -- http://gnuzilla.gnu.org
