Tools

The following is a list of tools and resources that will help with your web development process, as well as potentially debugging your Firefox browser add-ons.

Web development

Integrated into Firefox

Tools and resources that are built into Firefox and are in heavy development.

Page Inspector: A useful tool that shows you the HTML of the element you're currently pointing at with your mouse, in context.

Web Console: A console that lets you interactively output information, run JavaScript snippets, and experiment with your content.

Scratchpad: A text editor built into Firefox that lets you edit and run JavaScript code.
Style Editor: Lets you edit CSS styles for the current page in real time; in addition, you can toggle them on and off. A fantastic way to work in real time with the look and feel of web content!
JavaScript Debugger: Step through JavaScript code that is running in the browser (or even on a remote browser!) and watch variables to help track down bugs.
Responsive Design View: See how your designs will look on different screen sizes without changing the overall size of your browser window.
Using the Source Editor: The Source Editor isn't a standalone tool; instead, it's the underlying editor used by Scratchpad and the Style Editor. This article provides information common to all tools that make use of the Source Editor API.
Developer Toolbar: The Developer Toolbar provides a command-line interface to manipulate and work with the developer tools in Firefox, and buttons for quick access to the most commonly used tools.

Extensions

Firebug: Integrates a wealth of development tools to edit, debug, and monitor CSS, HTML, and JavaScript live in any web page (details)

DOM Inspector: A developer tool used to inspect, browse, and edit the DOM of documents - usually web pages or XUL windows.

Web Developer: Adds a menu and a toolbar to the browser with various web developer tools. (details)

Aardvark: Reveals DOM elements as you scrub the mouse over a page. (details)

More on Firefox Add-ons: Web development category in the Firefox Add-ons site

Validators

List of validators: Various validators for things like HTML, CSS, RDF, Firefox extensions, and many others.

Authoring Software

HTMLTidy: Tool used to fix or point out invalid HTML as well as improve the layout and indentation style of the resulting markup.

JavaScript

Error Console: Tool used for reporting errors in the application chrome and user-opened web pages. Also reports JavaScript-related errors and warnings, CSS errors and arbitrary messages from chrome code.

Venkman: Mozilla's JavaScript Debugger

JSLint: JavaScript Code Quality Tool

JSDoc: Tool used for generation of (usually HTML) documentation from code comments (similar to JavaDoc).

DOM

DOM Inspector: Developer tool used to inspect, browse, and edit the Document Object Model of documents

MODI: Mouseover DOM Inpector is a bookmarklet that allows you to view and manipulate the DOM of a web page simply by mousing around the document.

Localization

Localization and Leveraging Tools: Tools and resources to help localize and translate your extensions to other languages.

Accessibility

Policy

WAI-ARIA Overview: An overview of WAI-ARIA, the Accessible Rich Internet Applications Suite; a set of documents that lay out how to make Web 2.0 sites and applications more accessible.

Testing Tools

AChecker: AChecker is a tool that allows you to type in a url and test for accessibility.
W3C complete list of tools: A Complete list of web accessibility evaluation tools

A more comprehensive list of accessibility resources, see: Fluid Project accessibility resources and ARIA

Tree and branch management

cross-commit script: Concurrent Versions System(CVS) script that allows a single patch to be easily checked in on multiple branches.

User profile tools

Multiple Firefox profiles

Navigating the Mozilla Codebase

MXR (if you use vim, mxr-vim speeds up the process)

DXR

Profile Manager