FlashFirebug (now Flash Debugger) gives you the ability to debug any AS3 Flash on the web. Inspect the DisplayList, change properties on the fly and see the warnings, errors and traces directly in the extension. Use the inspector to hone in on elements, transform objects on the fly and use the console to run AS3 commands.

SWF Inspector

FlashFirebug has a Firebug-like inspector that allows you to easily locate any element inside the DisplayList without needing to examine the tree. You can also right click an element inside the SWF to quickly locate it in the tree.

Firebug-like Inspector

Simply click the FlashFirebug inspect button and hover over elements inside the Flash.

Firebug alternative right click

You can alternatively right click the elements inside the flash file and click "Inspect Element".