if you are a Web developer, designer who work on web sites, this tools is very good for you.
this tools is very usefull for us on test and tweak your web script code..
Just the way you like it
Firebug is always just a keystroke away, but it never gets in your way. You can open Firebug in a separate window, or as a bar at the bottom of your browser. Firebug also gives you fine-grained control over which websites you want to enable it for…
F12 is your friend
The F12 key is undoubtedly the fastest way to get to Firebug. Train yourself to hit that key when it’s time to go debugging. If you’re the big screen type, use Control+F12 (or Command+F12 on Mac) to open Firebug in a separate window.
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.
View source live
See changes highlighted
Firebug highlights changes to the HTML in yellow immediately when they occur. If you want to spy even closer, you have the option to also scroll every change into view, so you won’t miss a thing.
Find elements with the mouse
Something in your page doesn’t quite look right and you want to know why. There’s no faster way to get answers than to click the “Inspect” button on Firebug’s toolbar and then prepare for immediate gratification. As you move around the page, whatever is beneath your mouse will be instantly revealed within Firebug, showing you the HTML and CSS behind it.
Monitor network activity
Quickly find errors
Explore the DOM