I'm having an issue with a Flash/Flex erroring in Firefox but not IE. I need to see the error that the Flash/Flex app is getting from the ASP.NET app. Is there any way to debug the response that Flash/Flex is getting?
Install the Debug version of the FlashPlayer for Firefox. Maybe this is already enough and an Error might pop up.
If not use the FlexBuilder and debug the Flex application. If you don't have a license for the FlexBuilder you may also use the Evaluation licence.
Remember that you can also observe the network traffic from firebug, even for flash apps.
Depending on how you are making calls to your ASP.net app - you could use something like LiveHTTPHeaders to see the url that your flex app is call and see what response is being sent back from the server (i.e. 200, 404, 503, etc...)
Also - you could use something like Charles to do the same thing but actually see the full response from the server (be it images, text, xml...) and not just the header info.