The Thögosphere

Web Developer Toolbar
Adds a menu and a toolbar with various web developer tools.

FireBug
Edit, debug, and monitor CSS, HTML, and JavaScript live in any webpage.

YSlow (For FireBug)
Analyzes web pages and tells you why they’re slow based on Yahoo’s rules for high performance web sites.

ColorZilla
Advanced Eyedropper, ColorPicker, Page zoomer.

Update (12/07/08 - 15:08)

HTML Validator
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 in the status bar when browsing. The details of the errors are seen when looking the HTML source of the page.

Clear Cache Button
Adds a clear cache toolbar button. It clears your cache :-) [Thanks Jon!]

Update (15/08/08 - 00:40)

XRAY
Not really a Firefox add-on, but a bookmarklet (which can be used in other browsers as well). Use it to see the box model for any element on any web page.

Pixel Perfect
An add-on that allows web developers to easily overlay a web composition over top of the developed HTML. Switching the composition on and off allows the developer to see how many pixels they are off while in development.

Update (29/10/08 - 11:44)

Aardvark
As you glide the mouse over the page, you will see a red rectangle framing each element under the cursor. You will also see a little yellow caption showing the HTML element type and its class or id if they exist. Pressing certain keys on the keyboard will do various things, as descibed in the Keystroke list below.

Any webdesigner add-ons you can’t live without? Leave a comment!