One thing that I found indicative of the "depth" of Firebug is the very simple context menu item when you right-click a URL in the Network tab: "Copy as Curl command" (the text may read differently, this is from memory). It creates a clipboard entry which contains a "curl" command line with all the cookie/header/etc. data that would be sent from your browser if you were to request that URL (via Ajax or whatever). You paste that into your terminal and you can keep retrying indefinitely without having to click/interact with the web page at all. Hugely useful if you're debugging the server side of a client->server interaction.