Firebug is a add-on that is available with firefox browser and a firefox-lite with chrome.
So what is so great about the Firebug?
1. Firebug lets you inspect each HTML elements (like <a>, <div>, <table> ...) and when you inspect the element then the element, its margin, padding, border gets highlighted.
2. On inspecting a element, you can also the view the CSS used to style that part. You can also manipulate and tweak the css to view the result on the fly.
3. To see what all resources are loaded when a page is requested.To find out the time taken and the order in which the resource is loaded. To find out the status, url, domain, ip, size and Header info.