The Web Developer extension adds various web developer tools to the browser.
Use tamperdata to view and modify HTTP/HTTPS headers and post parameters...
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.
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)
A must have tool for Web Developers and Testers to Manage and monitor web cookies.
Export / Backup
Import / Restore
Available in English, French, Spanish, German and Italian languages.
Collections are groups of related add-ons that anyone can create and share.Explore Collections