One Click to Start Firebug
After you install Firebug you will see a gray 'Firebug' in the Firefox status bar (along the bottom of the window). Click on the Bug to start. Firebug will open in the bottom of your web page.
Inspect and edit HTML
As soon as you open Firebug you will see the Web page HTML,CSS styling, and object model. Three tabs -- HTML, CSS, and DOM -- activate as soon as you open Firebug. If you are a Web designer, this may be all of the Firebug you need. as Firebug makes it simple to find HTML elements buried deep in the page. Once you've found what you're looking for, Firebug gives you a wealth of information, and lets you edit the HTML live.
Visualize CSS metrics
When you’re editing CSS and the CSS boxes aren't lining up correctly it can be difficult to understand why. Let Firebug be your eyes and it will measure and illustrate all the offsets, margins, borders, padding, and sizes for you.
Quickly find errors
Explore the DOM