Developer tools for Firefox

FireBug lets you explore the far corners of the DOM by keyboard or mouse. All of the tools you need to poke, prod, and monitor your JavaScript, CSS, HTML and Ajax are brought together into one seamless experience, including a debugger, error console, command line, and a variety of fun inspectors.
Some Fun Features

Logging for web pages
Web developers have suffered with "alert debugging" for centuries. If you enjoy clicking the "Ok" button 40,000 times a day, FireBug is not for you. Otherwise, you will enjoy having the ability to log messages from JavaScript in your web page directly to the FireBug console.

JavaScript Debugging
Want to stop your JavaScript and step through it line by line? No need to fire up a big fat debugger. FireBug includes a simple lightweight way to set breakpoints in your scripts and examine every step of execution.

Errors at your fingertips
Nobody likes to see errors in their web pages, but with FireBug you not mind as much. FireBug adds a little icon to the Firefox status bar which tells you if your page is busted. Open up the FireBug panel and you are looking at the errors that occurred in that page, and only that page. No more digging through the muddy pile of errors in the JavaScript Console window.

Inspectors galore
Elements and styles and events, oh my. There is a lot going on behind the curtain of a web page. FireBugs inspectors let you stroll lazily through the DOM using the familiar hyperlink model of the web. If you see an object reference, you can click it to inspect it.

Command line
If you enjoy "alert debugging" then you will probably also enjoy typing "javascript: " urls in the location bar all day. If not, then you will enjoy using FireBugs JavaScript command line instead.
Ajax Request Spy If you feel hip every time you fire off an XMLHttpRequest on your Ajax website, FireBug will secure your place amongst the avante garde. The FireBug console can log all Ajax request traffic as it happens, and allows you to inspect the responses.

Live Editing
It is no fun just looking at the DOM. FireBugs inspectors let you edit some parts of the DOM. More editing features to come in a future version.

FirePHP
FirePHP enables you to log to your Firebug Console using a simple PHP method call.
All data is sent via response headers and will not interfere with the content on your page.
FirePHP is ideally suited for AJAX development where clean JSON and XML responses are required.

Web Developer
The Web Developer extension adds a menu and a toolbar to the browser with various web developer tools. It is designed for Firefox, Flock and Seamonkey, and will run on any platform that these browsers support including Windows, Mac OS X and Linux.