I just found a new debugger for Adobe Flex applications. It pretty much works along the same lines as Firebug does for debugging web pages. The debugger is called Kap Inspect.  To use it in your Flex apps, all you have to do is include the SWC file in your project, add the namespace, and a single MXML tag and you are up and running. 

 With your Flex app running in the browser just hit Ctrl + Alt + F12 and a debugger window will pop-up that allows you to inspect events process, check the design and the styles, manipulate objects properties and more.

Here is a demo you can try out. One thing I really like is that I can use it even without running my app in Debug mode!  If you do any Flex development I suggest you check it out!

Adobe Flex