YSlow analyzes web pages and suggests ways to improve their performance based on a set of rules for high performance web pages. YSlow is a Firefox add-on integrated with the Firebug web development tool. YSlow grades web page based on one of three predefined ruleset or a user-defined ruleset. It offers suggestions for improving the page's performance, summarizes the page's components, displays statistics about the page, and provides tools for performance analysis, including Smush.it™ and JSLint. 

For more info go to: http://developer.yahoo.com/yslow/

StumbleUpon Discover photos, videos, and web pages recommended by friends and people sharing your interests. StumbleUpon learns what you like, so you only see what's interesting to you from across the web. 

If you want to joint go to: http://www.stumbleupon.com/

Personalized Minimized Allows users of Google's personalized homepage to minimize blocks of content.

For download: http://www.kevinfreitas.net/extensions/personalize-minimized/

PictureThis A word is worth a thousand images as you will see when you highlight a word or words on a page then use PictureThis to see what Google Images sees. It's a whole new way of adding instant context to the web.

For more info visit: http://www.kevinfreitas.net/extensions/picturethis/

LinkChecker Check webpage links at a glance with simple color coding. Ditch those massive listings of bad links that provide no context and add LinkChecker to your arsenal of web development tools today.

For download go to: http://www.kevinfreitas.net/extensions/linkchecker/

Measure IT, draw a ruler across any webpage to check the width, height, or alignment of page elements in pixels.

For download go to:  http://www.kevinfreitas.net/extensions/measureit/

CSSViewer With this extension you are able to view the css properties.Ever been on a site that had some nifty CSS tricks that you loved and wished you knew how they did it?

For download go to: https://addons.mozilla.org/en-US/firefox/addon/cssviewer/

SEO for Firefox! Want to know why Google or Yahoo! ranks pages? If so this is the Firefox extension for you. SEO for Firefox pulls in many useful marketing data points to make it easy get a holistic view of the competitive landscape of a market directly in the search results.

If you want to download, see video tutorial etc visit: http://tools.seobook.com/firefox/seo-for-firefox.html

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.

The extension is based on Tidy and OpenSP. Both algorithms were originally developed by the Web Consortium W3C. And now extended and improved by a lot of persons. Both algorithms are embedded inside Mozilla/Firefox and makes the validation locally on your machine, without sending  HTML to a third party server.

If you want to download, go to: http://users.skynet.be/mgueury/mozilla/

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 640x480, 800x600, 1024x768, 1280x800, 1280x1024, 1600x1200 resolutions.
Resize the current page via the Context menu, Tool menu or Toolbar button.

To add Firefox visit:https://addons.mozilla.org/en-US/firefox/addon/1985/

Press PrintScreen and can't grab the entire webpage because it is longer than the browser window?  You need FireShot!

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

Screenshots can be saved to disk (PNG, GIF, JPEG, BMP), printed (NEW), copied to clipboard, e-mailed and sent to external editor for further processing.

A free image hosting web-server is also provided.

To add firefox visit:https://addons.mozilla.org/en-US/firefox/addon/5648/

For more info visit the official website producer: http://screenshot-program.com/fireshot/

With ColorZilla you can get a color reading from any point in your browser, quickly adjust this color and paste it into another program. You can Zoom the page you are viewing and measure distances between any two points on the page. The built-in palette browser allows choosing colors from pre-defined color sets and saving the most used colors in custom palettes. DOM spying features allow getting various information about DOM elements quickly and easily. And there's more...

For download visit: http://www.colorzilla.com/firefox/

FireFTP is a free, secure, cross-platform FTP client for Mozilla Firefox which provides easy and intuitive access to FTP servers.

For download visit: http://fireftp.mozdev.org/

Font Finder is designed for any web developer / designer that needs to find out the CSS information of any text on the page.

Simply highlight any text element, right-click, and choose font finder. An alert box will appear with all of the styling information you could require including font-size, font-family, color (in both hex and RGB), line-height, letter-spacing, and much more! In addition, you are not told only the CSS font stack, but exactly which font from the stack is currently being rendered.

For download visit mozilla addons page: https://addons.mozilla.org/en-US/firefox/addon/4415/

IE Tab, an extension from Taiwan, embeds Internet Explorer in a Mozilla/Firefox tab.
This extension is derived from the famous extension IE View, but much different.
While IE View always open IE-only pages in a seperate Internet Explorer window, IE Tab can view them in a Mozilla/Firefox tab.

For more info and download visit: http://ietab.mozdev.org/

Greasemonkey is a Firefox extension that allows you to customize the way webpages look and function. Hundreds of scripts are already available for free. And if you're the tinkerer sort, you can also write your own.

For more info please visit: http://www.greasespot.net

Seoquake is a powerful tool for Mozilla Firefox, aimed at helping web masters who deal with search engine optimization and internet promotion of web sites. Seoquake allows user to obtain and investigate many important SEO parameters of the internet project under study on the fly.
Project Seoquake is being constantly polished and enhanced. All SERP changes are tracked down and taken into account. Update your version regularly.

For tutorial, download, etc visit: http://seoquake.com/

The Web Developer extension adds various web developer tools to a browser. The extension is available for Firefox and Chrome, and will run on any platform that these browsers support including Windows, Mac OS X and Linux.

For download, more info etc visit: http://chrispederick.com/work/web-developer/

Firebug 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. Firefox has a "View Source" window, but it doesn't show you what the HTML source really looks like once it has been transformed by JavaScript. Firebug's HTML tab shows you what the HTML looks like right now.

Firebug's CSS tabs tell you everything you need to know about the styles in your web pages, and if you don't like what it's telling you, you can make changes and see them take effect instantly.

For free download, documentation or update visit: http://getfirebug.com/

Measurement – Quantcast’s free audience measurement for marketers provides detailed user demographics and deep audience insights at the campaign, site and conversion level. This information includes traffic, demographics, geography, site affinity and categories of interest. Through our intuitive reporting interface, you will receive audience analysis at the campaign, publisher, placement, creative, site and event level.

For more information visit: http://www.quantcast.com