DOM Inspector is a tool that can be used to inspect and edit the live DOM of any web document or XUL application. The DOM can be navigated using a two-paned window displaying a variety of different views on the document and all nodes within.
Scratchpad places an add-on bar button that open Firefox Scratchpad qucikly.
Open Dom Inspector at this node from Inspect on context menu. Dom Inspector must already be installed. Handy for checking both page for web developers and chrome for extension...