Aardvark (Developer Page)

Powerful and user-friendly selector utility for selecting elements and doing various actions on them. It can be used for cleaning up a page prior to printing it (by removing and isolating elements), and for web development.

CLEO (Developer Page)

CLEO allows you to organize individual extensions/themes by combining them together into a single, installable file. This file can be used to install or restore a collection of extensions and/or themes.

Firebug (Developer Page)

Firebug integrates with Firefox to put a wealth of development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.

Google Notebook

I used a local tool to store various code snippets I needed over and over and sometimes even Google mail if I used them on a different computer. Well, I never ever had them on the computer I needed them until a friend pointed me to Google Notebook and it really speeds-up my copy & paste experiences ;)

Greasemonkey (Developer Page)

Allows you to customize the way a webpage displays using small bits of JavaScript. Hundreds of scripts, for a wide variety of popular sites, are already available at userscripts.org.
You can write your own scripts, too. Check out wiki.greasespot.net to get started.

Hammerhead (Developer Page)

Hammerhead adds a tab to Firebug for measuring the load time of web pages. Requires Firebug.

HTML Validator (Developer Page)

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 extension is based on Mozillas built-in support for Tidy and OpenSP.

HttpFox (Developer Page)

HttpFox monitors and analyzes all incoming and outgoing HTTP traffic between the browser and the web servers.

Live HTTP Headers (Developer Page)

View HTTP headers of a page and while browsing.

Modify Headers (Developer Page)

Add, modify and filter http request headers. You can modify the user agent string, add headers to spoof a mobile request (e.g. x-up-calling-line-id) and much more. Take a look at the help tab of the Modify Headers window.
For an alternative, check Tamper Data

Nightly Tester Tools (Developer Page)

This extension adds a few extras useful to those that regularly test nightly builds of Firefox, Thunderbird, Sunbird and Toolkit Seamonkey (Suiterunner). The following is a brief list of the extension’s features:
Extension compatibility fixing, Titlebar customisation, Build ID retrieval, Screenshots, Breakpad information, Restoring tabs from previous session & Leak log analysis.

Operator (Developer Page)

Operator lets you combine pieces of information on Web sites with applications in ways that are useful. For instance, Flickr + Google Maps, Upcoming + Google Calendar, Yahoo! Local + your address book, and many more possibilities and permutations. All of these scenarios are possible due to Microformats, an emerging standard for injecting semantics into HTML.

Pencil (Developer Page)

Pencil is a free and opensource Firefox add-on to do GUI prototyping and simple sketching within Firefox.

Pixel Perfect (Developer Page)

Pixel Perfect is a Firefox/Firebug extension that allows web developers and designers to easily overlay a web composition over top of the developed HTML. Requires Firebug.

Platypus (Developer Page)

Platypus is a Firefox extension which lets you modify a Web page from your browser — “What You See Is What You Get” — and then save those changes as a Greasemonkey script so that they’ll be repeated the next time you visit the page. Requires Greasemonkey.

Poster (Developer Page)

A developer tool for interacting with web services and other web resources that lets you make HTTP requests, set the entity body, and content type. This allows you to interact with web services and inspect the results. After you install it, just go to View->Sidebar->Poster to open the tool as a sidebar. After that, try a GET or POST on your favorite web service or site.

SearchStatus (Developer Page)

Display the Google PageRank, Alexa rank and Compete ranking anywhere in your browser, along with fast keyword density analyser, keyword/nofollow highlighting, backward/related links, Alexa info and other SEO tools.

Selenium IDE (Developer Page)

Selenium IDE is an integrated development environment for Selenium tests. It is implemented as a Firefox extension, and allows you to record, edit, and debug tests. Selenium IDE includes the entire Selenium Core, allowing you to easily and quickly record and play back tests in the actual environment that they will run.
For an alternative, check Molybdenum.

Smush it (Developer Page)

Optimizing images by hand is time consuming and painful. Smush it does it for you.

Total Validator (Developer Page)

Provides true HTML validation (HTML 2.0 to XHTML 1.1) using the official DTDs, plus added attribute checking. So you no longer have to put up with the limitations of the W3C validator (no type checking) and tools like HTML Tidy which interpret the standards incorrectly (they don’t use the official DTDs). Also performs accessibility validation (WCAG, US-508), broken link checking, spell checking (5 languages), and takes screenshots using 32 browsers on Windows, Linux, and OS/X.

UrlParams (Developer Page)

Shows you the GET and POST parameters of the current website in the sidebar. You can alter their values, add new parameters, switch get/post and more. Might not be compatible with Firefox 3.0.x.

YSlow (Developer Page)

YSlow analyzes web pages and tells you why they’re slow based on Yahoo’s rules for high performance web sites. Requires Firebug.

Web Developer (Developer Page)

A well-known add-on and must-have for every web developer. Adds a menu and a toolbar with various web developer tools.