Mozinet's Web Developer Toolbox / les extensions pour développeurs Web de Mozinet
The Web Developer extension adds various web developer tools to the browser.
Advanced Eyedropper, Color Picker, Gradient Generator and other colorful goodies...
View HTTP headers of a page and while browsing.
Draw a ruler across any webpage to check the width, height, or alignment of page elements in pixels.
YSlow analyzes web pages and suggests ways to improve their performance based on a set of rules for high performance web pages.
http://yslow.org is the official open source website.
FontFinder is created for designers, developers and typographers. It allows a user to analyze the font information of any element on a page, copy any piece(s) of that information to the clipboard, and perform inline replacements to test new layouts.
Color tools for web development.
Color picker and eyedropper + saving colors and trying out colors with drag and drop.
HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla. The number of errors of a HTML page is seen on the form of an icon
For Firefox 23+, you need 0.958 (under review)
CodeBurner is a Firefox add-on that integrates with Firebug, to extend it with reference material for HTML and CSS.
It adds a tab to the Page Info dialog box, which shows the cookies belonging to the current page.
Dust-Me Selectors is development tool to remove unused CSS selectors.
Open panel to left, bottom or right (left,middle or right click) with current URL from Split on context menu (right click on page) of the tab's page.