FlashFirebug is a Firefox extension that allows developers to debug AS3 flash files on the web. The main purpose of FlashFirebug is to make Flash debugging as easy as HTML and javascript debugging with an interface derived from Firebug and familiar to developers. This will fill in the shortcoming of Flash debugging which is not available in Firebug. With this plugin, Firebug will cover the needs of all web developers.

Besides Firebug, it requires Flash Player content debugger 10 or higher and enables us to inspect display list, change properties and see the warnings, errors + traces for any Flash file.
It can also work with multiple Flash files on a web page and display the elements inside the file as a directory tree.
The extension is designed to perform fast and, optionally, it can be disabled to be used only when it is needed.
Requirements: Firebug & Flash Player content debugger 10+
Compatibility: Firefox
Website: http://www.o-minds.com/products/flashfirebug