Firefox + Firebug is your friend.
https://addons.mozilla.org/firefox/addon/1843
Also, you may want to check out YSlow & Web Developer too:
https://addons.mozilla.org/firefox/addon/5369
https://addons.mozilla.org/firefox/addon/60
With these tools, you can debug sites like a pro! Which is very helpful when you are trying to figure out how to optimize your site.