I recently encountered some issues with trying to debug a flash application that posted an XML packet to a URL. I first tried FireBug, it didn’t seem to be picking up the XML that was posted. After a quick Google search, I ran across a post describing how to use Fiddler (an IE based tool) with Firefox.
Check out the simple setup steps:
Bonus! I was able to view the raw header and the XML data that was being posted.
NOTE: I have heard of some issues with having to have Fiddler open to browse in Firefox, but haven’t encountered it.