Whenever I develop in Flex, I’ve always used ServiceCapture. It is a commercial product that inpects all type of traffic including Flex’s AMF, a binary format based loosely on the Simple Object Access Protocol (SOAP). ServiceCapture runs about $34 per seat. It allows me to see what traffic is being sent back and forth between my ColdFusion Server and Flex for debugging and QA purposes.
I was made aware today of a new player in town that integrates into FireBug for FireFox. It’s currently experimental, but seems to work nicely. Check it out.