Firebug and Network Monitoring
Watch the timeline unfold
Each file in the Net tab has a bar which shows you when the file started and stopped loading relative to all the other files. The bars will teach you things you didn't even know. This will help you tune the order of files in page so that the user spends less time waiting for things to show up. Vertical line across bars represents an event (e.g. blue line == DOM content loaded)
Break it down by type
Cached or not cached
Not all network requests are equal - some of them are loaded from the browser cache instead of the network. Firebug provides status codes for every request so you can quickly scan and see how effectively your site is using the cache to optimize page load times.
Examine HTTP Headers
HTTP headers contain all sorts of interesting information like the mime type of the file, the type of web server, caching directives, the cookie, and lots more. To see the HTTP headers, just click the arrow to the left of each request to expand it.
The phenomenon that is Ajax revolves around a little thing called XMLHttpRequest. It's not much fun to send these requests out and get no visual feedback about their travels. Firebug shows you each XMLHttpRequest, both in the Net tab and the Console tab, along with the text that was posted and the text of the response.
console.timeStamp() to create visual time stamps drawn over the graphical time line.