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 hierarchy can be navigated using a two-paned window that allows for a variety of different views on the document and all nodes within. This screenshot shows a typical inspecting session.
Launching DOM Inspector
From within Mozilla
You will find it under the Tools > Web Development menu in any Mozilla window.
From the command line
Run: /path/to/mozilla -inspector [url]
From the Mozilla sidebar
First you must install the sidebar, via Edit > Preferences > Advanced > DOM Inspector. After you have done that, simply open up the inspector panel and visit a website. It's that simple!
Using DOM Inspector
Start playing with it and you'll get the hang of it. Formal documentation is still under development, but there is a short FAQ available.