Firebug

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

Firebug is always just a keystroke away, but it never gets in your way. You can open Firebug in a separate window, or as a bar at the bottom of your browser. Firebug also gives you fine-grained control over which websites you want to enable it for.

Firebug makes it simple to find HTML elements buried deep in the page. Once you’ve found what you’re looking for, Firebug gives you a wealth of information, and lets you edit the HTML live.

The Document Object Model is a great big hierarchy of objects and functions just waiting to be tickled by JavaScript. Firebug helps you find DOM objects quickly and then edit them on the fly.

Web Developer

The Web Developer extension adds a menu and a toolbar to the browser with various web developer tools. It is designed for Firefox, Flock and Seamonkey, and will run on any platform that these browsers support including Windows, Mac OS X and Linux.

Windows Resizer

The Browser Window Resizer is useful for testing different screen sizes. It accurately resizes your browser so you can test to see what a web page looks like in all of the standard resolution sizes.

Supports the 640×480, 800×600, 1024×768, 1280×800, 1280×1024, 1600×1200 resolutions.

Resize the current page via the Context menu, Tool menu or Toolbar button.

FireShot

FireShot is a Firefox extension that creates screenshots of web pages (entirely or just visible part).

Unlike other extensions, this plugin provides a set of editing and annotation tools, which let users quickly modify web captures and insert text annotations and graphical annotations. Such functionality will be especially useful for web designers, testers and content reviewers.

The captures can be:

• uploaded to FREE public screenshot hosting
• saved to disk (PNG, GIF, JPEG, BMP)
• printed
• copied to clipboard
• e-mailed
• sent to configurable external editor for further processing.

MeasureIt

Draw out a ruler to get the pixel width and height of any elements on a webpage. Link here.