Troubleshooting web browser applications is a pain! I'm sure most of you that are agreeing with me have been in a situation where you have absolutely no idea what is getting passed from the web browser to your server. Especially when you are doing something convoluted and special (most commonly refered to as a hack!). Anyways I work in a mostly Microsoft shop, and all of these dev's live and die by Fiddler. Call me crazy, or maybe I just don't understand the use, but what is the difference between proxying all of your connections through Fiddler, and sniffing your packets? I guess one could REALLY step through code or something with Fiddler. Or I'm completely mistaken and Wireshark is proxying! I plainly don't see the point in it. So what I normally do is fire up Wireshark, and for HTTP based applications I use the following filter:
http.request or http.response and ip.addr == XXX.XXX.XXX.XXX
Where "XXX.XXX.XXX.XXX" is the IP address of your main networked network interface card (NIC). It works great! I hope this helps someone out there.