The User Agent Switcher extension adds a menu and a toolbar button to switch the user agent of a browser.
The Web Developer extension adds various web developer tools to the browser.
FireFTP is a free, secure, cross-platform FTP/SFTP client for Mozilla Firefox which provides easy and intuitive access to FTP/SFTP servers.
Check out my new project, FireSSH!
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.
Cookies manager to view, edit and create new cookies. It also shows extra information about cookies, allows edit multiple cookies at once and backup/restore them.
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.
Firefox OS Simulator is a test environment for Firefox OS. Use it to test your apps in a Firefox OS-like environment that looks and feels like a mobile phone. After installing it, go to Web Developer > Firefox OS Simulator to access its features.
Add, modify and filter the HTTP request headers sent to web servers. This addon is particularly useful for Mobile web development, HTTP testing and privacy.
Wappalyzer is a browser extension that identifies software on websites.
Color tools for web development.
Color picker and eyedropper + saving colors and trying out colors with drag and drop.