- ColorZilla - Advanced eyedropper, color picker, page zoomer and other colorful goodies. This is pretty useful for web designers. You can get the color code on areas of the webpage, much like the Photoshop eyedropper.
- DownThemAll! - The mass downloader for Firefox. Very handy if you download lots of stuff from the Internet. Makes downloading faster and much more organized. A lot of options available for customizing your downloads.
- Firebug - Web development evolved. I think the most comprehensive debugging tool for web developers. Analyze the internal code of a page (html, css, javascript, dom) and more useful developer tools.
- FireFTP - FTP client for Mozilla Firefox. I personally use this. It has replaced my default FTP client.
- Fission - Places the progress bar in the address bar, like Safari style.
- FoxClocks - Displays local times around the world in the status/toolbar. Supports virtually all timezones.
- FoxyProxy - Premiere proxy manager for Firefox. If you are a "stealthy" surfer and you usually change the proxies by going to the Options menu, this is a great add-on.
- MeasureIt - Draw out a ruler to get the pixel width and height of any elements on a webpage.
- PDF download - Allows you to choose what to do with a PDF file: download it, view it with an external viewer, or view it as HTML
- RankChecker - Track where your site ranks in Google, Yahoo, and Microsoft Live Search using the free Firefox Rank Checker. From SeoBook.com
- SiteLauncher - Opens websites using special keyboard shortcuts. This is specially useful if you want to remove the bookmarks toolbar to save screen real estate, but still want easy access to bookmarks. Pressing ctrl+space opens a pop-up of your set bookmarks.
- StumbleUpon Toolbar - StumbleUpon discovers web sites based on your interests, learns what you like and brings you more. Clicking the StumbleIt! button is quite addicting!
- Tab Mix Plus - Tab browsing with an added boost. If you are accustomed to using the ctrl+tab shortcut when changing tabs on other Windows applications, this will help you get the same effect on Firefox.
- TinyMenu - Replaces the standard menu with a tiny menu popup.
- Yet Another Smooth Scolling - Replaces scroll behavior with both mouse wheel and cursor key into fully customizable natural smooth one. This imitates the scrolling behavior of the Opera browser.
- Personas -
Personas are lightweight, easy to install and easy to change skins for your Firefox web browser. This one is really a must-try!
Video DownloadHelper - The easy way to download and convert Web videos from hundreds of YouTube-like sites. This works also for audio and picture galleries.
- Cooliris - Full-Screen, 3D -- Cooliris is simply the fastest and most stunning way to browse photos and videos from the Web or your desktop. Effortlessly scroll a "3D Wall" of your content from Facebook, Google Images, YouTube, Flickr, and hundreds more. You'll be stunned with the 3d interface of this image gallery add-on.
- GooglePreview - Inserts preview images (thumbnails) and popularity ranks of web sites into the Google and Yahoo search results pages. Very helpful when you do a lot of Googling!
- FoxTab - FoxTab is a 3D tabbed browsing extension. With 5 attractive layouts to choose from, fliiping between opened tabs becomes easy and enjoyable than ever. Use the grouping and filtering features, and start experiencing tabbed-browsing in a whole new way.
Any other cool Firefox Add-ons you can suggest? Place them in the comments section so others can also benefit!