Xmarks

• Formerly Foxmarks, Xmarks synchronizes your bookmarks across computers, via Xmarks servers. Incredibly useful, especially if you have lots of bookmarks and work on different computers. For example, everything you bookmark on your computer at work is seamlessly synced on your home desktop and/or laptop. Also, you can share folders of your bookmarks through Xmarks as webpages and/or RSS feeds. 20 million plus downloads!
• extension page | site

Windows Media Player Firefox Plugin

• Absolutely essential, as it allows you to watch Windows Media Players files in webpages in Firefox. Without it, no (WMP) videos for you. From Port 25, the Microsoft Open-Source group.
• extension page | site

FlagFox

• Displays the flag of the country in which the server hosting the website you are viewing is located. Useful for spotting spoof or phising sites, but of general interest as well. Note, the location is determined by the IP address of the server.
• extension page | site

Firefox Showcase

• Adds a small tab to the end of the browser tabs, which, when clicked, gives you a window with live preview of all your open tabs.
• extension page | site

Firebug

• Allows you to edit and preview HTML, CSS, Javascript and more of a live webpage. Get development and debugging tool for web designers.
• extension page | site

Drupal for Firebug

• Extends Firebug functionality to Drupal sites.
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Download Statusbar

• Replaces the default download pop-up window in Firefox with a low-profile status bar at the bottom of the window (kinda like downloading in Chrome).
• extension page | site

SmoothWheel

• Makes scrolling with a mouse wheel in Firefox smooth and sleek!
• extension page | site

FireFTP

• Integrates a full-featured FTP client into a Firefox tab.
• extension page | site

ColorZilla

• A eye-dropper type color picker that returns the RGB and HTML color values, as well as the screen XY coordinates for a single pixel. Another great web designer's tool.
• extension page | site

CoolIris

• Creates a 3D 'wall' in a Firefox tab (can be viewed in full-screen mode too), that provides a unique way of browsing photos, videos, and webpages.
• extension page | site

SearchStatus

• Queries Alexa, QuantaCast, and Google for traffic, keyword, and other SEO data for the site you are browsing.
• extension page | site