Have you come across Yahoo!’s YSlow plug-in for Firebug? It scores your page against their “Best Practices for Speeding Up Your Web Site”, and is handy for looking at the page’s external resources and relevant HTTP headers.
If you have used YSlow before you might be interested in the upcoming YSlow 2.0. I just came across a slideshow while looking for URLs for my comment above. The extensibility looks quite interesting.
http://www.slideshare.net/stoyan/yslow-20-presentation/