CSSMate - Firefox extension to edit CSS files.
CSS validator - Check the validity of your webpage using this CSS validator extension.
CSSViewer - See the CSS properties of page elements with this extension.
IE Tab - Designers and developers can view their CSS projects on Internet Explorer using this extension.
Style Sheet Chooser II - Users can pick and choose alternate style sheets for a website.
HTML Validator - Cool extension to validate web pages with HTML standards of W3C.
Link Checker - Check the links on a webpage for validity. Color to each link explains its current status.
Load Time Analyzer - View detailed graphs of the loading time of web pages in firefox. The graphs display events like page requests, image loading times etc.
View Dependencies - Find out all files that were loaded in a webpage with the view dependencies tab in page info.Clipmarks - Easily save portions of webpage with this extension.
Codetech - Excellent extension for web page designers, codetech is a web page editor with the look and feel of Dreamweaver.
ColorZilla - A nice extension for web designers and developers. Colorzilla helps users to select colors which can be adjusted and put in any other program.
FirePHP - Find out what PHP code in the backend does to pages on display in Firefox browsers.
Flash Switcher - Use this extension to avoid reloading between flash plugins.
Measureit - Users can calculate the measurement specifications of tables, paragraphs on internet.
Platypus - One of the best Firefox extensions for modifying a webpage. Users can modify a webpage, save their changes as a Greasemonkey script and their saved page would be loaded the next time they visit the page.
Poster - A must have tool for web developers enabling them to interact with web services and other web resources.
Professor X - Check out the contents of the head of page without getting into source code.
Screen grab - Capture screenshot of any webpage and save it as an image file with this extension.
TestGen4Web - Save the webpage activities users (entry of text, web page clicks) in xml file. These actions can later on be replayed and converted to scripts for automated test tools.
View Source Chart - View the source code of any webpage as Firefox sees it.
Web Developer - Add a menu of various web development tools with web developer extension.
X-Ray - Must have Firefox extension, it allows users to view webpage code with a right-click and shuffle between the webpage and its code easily.