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/