Daisuke Sakamoto, a web site developer using PHP, loves these add-ons. (This is for my backup.)
Blocks annoying video ads on YouTube, Facebook ads, banners and much more.
Adblock Plus blocks all annoying ads, and supports websites by not blocking unobtrusive ads by default (configurable).
The Web Developer extension adds various web developer tools to the browser.
Xmarks is the #1 bookmarking and open tab add-on. Keep your bookmarks, and open tabs backed up and synchronized across computers and browsers. Mobile phones with Premium access. Utilize LastPass to sync passwords!
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.
Get rid of that clumsy context menu, and search the web using up to 100 stylish Drag & DropZones
An extension that allows for the use of dynamic commands in Firefox.
Cookie Editor that allows you add and edit "session" and saved...
Firefox 3 comptible
Collections are groups of related add-ons that anyone can create and share.Explore Collections