The Inspector lets you select an element on a website or a XUL dialog for further investigation in Firebug.
The Inspector supports inspecting the contents of frames and iframes. It even works with elements of image maps and MathML formulas as well as XUL elements.
There are three ways to start the Inspector. Clicking the Inspect Button ( ) in the Panel Bar, using the shortcut (default: Ctrl+Shift+C or ⌘+Shift+C on Mac OS) for toggling inspection or right-clicking an element on a website and choosing "Inspect Element" from the context menu.
Simply hover the element you want to inspect with the mouse and click on it. In versions of Firebug prior to 1.7 this automatically switches to the HTML Panel and selects the appropriate element inside the Node View. This is still the default behavior in 1.7, but in this version a feature was added allowing other panels to support the inspector. This means using the Inspector while being at a specific panel, which supports inspection, will select the hovered element to give you more details about it.