Several extensions useful to someone performing web development using PHP (potentially including AJAX).
The Web Developer extension adds various web developer tools to the browser.
Use tamperdata to view and modify HTTP/HTTPS headers and post parameters...
View HTTP headers of a page and while browsing.
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)
FirePHP enables you to log to your Firebug Console using a simple PHP method call.
Not using Firebug 1.9+ on Firefox 8+? See http://firephp.org/
CodeBurner is a Firefox add-on that integrates with Firebug, to extend it with reference material for HTML and CSS.
Dust-Me Selectors is development tool to remove unused CSS selectors.
Collections are groups of related add-ons that anyone can create and share.Explore Collections