Firefox addons which are useful for search engine optimization while searching / browsing.

Foxy SEO Tool – Foxy SEO Tool is a free, open source toolbar that provides quick and easy access to a multitude of search engine functions such as Google Site, Yahoo! Site Explorer, Live Fromlinks. Also included are famous web traffic analysis sites such as Alexa, Compete, Quantcast, popular statistic aggregators like Quarkbase and WebSiteOutlook. You can check a site’s listing in important directories like Dmoz and Yahoo! Directory, buzzing Web 2.0 sites such as Technocrati, Wikipedia, Digg, Delicious and links to useful network functions like Whois, Geotool, Internet Archive, etc.

RankQuest SEO Toolbar – This toolbar provides you quick access to more than 30 intuitive SEO tools. Alexa Rank and Page Rank provided by Alexa and Google respectively ensures the popularity of the site. Once you download and install the SEO Toolbar you are only one or two clicks away from carrying out most of your day to day SEO operations.

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Web Developer

The Web Developer extension adds a menu and a toolbar with various web developer tools.
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ColorZilla

Advanced Eyedropper, ColorPicker, Page Zoomer and other colorful goodies.
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.
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Recommended Search Engines Add-ons for Firefox

1. Context Search – Expands the context menu’s ‘Search for’ item into a list of installed search engines, allowing you to choose the engine you want to use for each search.

2. WikiLook – WikiLook is a Wikipedia & Dictionary look up. Fast, light, open source tool that define words for you without need to open new tabs or extra windows. Ctrl or Shift or Alt + RightMouseButton click. More then 70+ different …

3. Google Toolbar – Take the power of Google with you anywhere on the Web..

4. CoolPreviews – Browse faster — Preview links and images without leaving your current page or tab. Just mouseover any link, and a separate preview window immediately appears to show you the content. (TIP: The fastest way to view many links on a …