MOZDEV -- One of the things I miss about riding the 69 bus from Citywest to Dublin City is overhearing developers talk about browsers. Some of those Citywest developers are using Daniel Savard's Live HTTP Headers inside Mozilla Firefox to debug web applications. You can also see the kind of web server used by the remote site and see the cookies sent by the remote site.
You can download and integrate this functionality to your installation of Firefox. Once installed, it adds a "Live HTTP Headers" tab in the pull-down "Tools" menu of Firefox. You can display this tool as a slidebar and slide its elevator tab down as you browse. You can save your header activity and review it later. The documentation says you can edit request headers and replay the URL but I haven't done that.
This is useful stuff.
Daniel Savard -- "Live HTTP Headers Project"
I learned this from a referrer.