Inspect and edit HTML
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.
Tweak CSS to perfection
Firebug’s CSS tabs tell you everything you need to know about the styles in your web pages, and if you don’t like what it’s telling you, you can make changes and see them take effect instantly.
Visualize CSS metrics
When your 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
and many more useful functions…