Firebug integrates with Firefox to put a wealth of development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page...
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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.
Extract the color schemes of websites from their images and CSS, and try out color schemes using basic color tools.
HttpFox is an addon for Mozilla based browsers (Firefox, Flock, ...) that monitors and analyzes all incoming and outgoing HTTP traffic between the browser and the web servers.
It aims to bring the functionality known from tools like HttpWatch or IEInspector to the Firefox browser.
Information available per request includes: