Only work for AS3's swf, and Firefox need install Debugger Version Flash Player 9(+). and it sometimes may crash Flash Player, because some unexcept reason from target swf, so I recommend update your Firefox to 3.6.4(+), as it's release note: "If a plugin crashes or freezes, it will not affect the rest of Firefox. You will be able to reload the page to restart the plugin and try again. "
and send feedback to me if you find any problem or idea. thanks very much.
FlashInspector can work with FlashFirebug (another great addon for Flash Developers), you can inspect DisplayObject in Firebug's panel.