Flash Firebug is a great tool for debugging Actionscript 3 SWF files, similar to the way regular Firebug helps debug Javascript and CSS when using Firefox.

But the current version of Flash Firebug, 3.0.3, has a bug that causes it to display this error:

No SWF files with AS3 detected on this page. You could try reloading the page.

This can happen even when there definitely IS an Actionscript 3 swf on the current page. To resolve the problem, install any later version than 3.0.2, available from this site:

FlashFirebug Version History