I used to rely on the Firefox Plugin Tamper Data to get the nitty gritty deatails of http requests. Firebug has a feature that is similar as well (the net tab). But I find it buggy. Recently, a fellow developer introduced me to the HttpFox Firefox plug-in, and it is awesome!
I now use it all the time.
The only caveat, is only start it when you need it, and stop it when you are done. Leaving HttpFox running and capturing for long periods of time slows Firefox a lot.
Check it out: http://code.google.com/p/httpfox/