There are many times when it is really handy to see exactly what the browser is sending to the server and vis versa. Say when you are sure you are setting a cookie but it doesn’t seem to be working.
You can do it the old fashioned way, with a packet sniffer but quite frankly that is just to painful. Enter the HTTPLiveHeaders extension for firefox.
It shows you everything that goes between the browser and the server – just perfect for getting to the root of the communication issue.