Adblock Plus - Block ads Cutemenus: Crystal SVG - Makes your menus look much nicer by adding icons to all actions (for firefox 3.0 or lower) Menu Icons Plus - Enhance your menus (for firefox 3.5 or higher) Download Statusbar - Get rid of the annoying download popup and put a miniature version of it in your statusbar Foxytunes - Control your media player from the statusbar Skipscreen - Skip annoying or unnecessary screens for download sites Smarterfox - Enhances firefox in various ways (like by automatically loading the 'next page' on multi page documents such as forums and search results) Read It Later - Prevent your bookmarks menu from becoming cluttered by marking documents to be read later ColorZilla - Advanced eyedropper, page zooming utility and other goodies (good for web designers and other people that work with color)
Enhances Amazon.com with a compilation of user scripts and styles. All scripts copyright their original authors. Click on the script homepage in the Help tab for more information.
A compilation of useful Greasemonkey scripts for Flickr. All scripts copyright their original authors. Click on the script homepage in the Help tab for more information.
Enhances Gmail with a compilation of user scripts and styles. All scripts copyright their original authors. Click on the script homepage in the Help tab for more information.
BugMeNot 2.2
Bypass compulsory web registration with the context menu via www.bugmenot.com.
Noscript is nice.
Blocks applets and several other controls on webpages.(Flash, Java applets, those annoying hidden controls that try to install a virus, etc.) You can toggle them to always allow, temp allow, or block. Great for security purposes, and if you only visit a few sites, after a while you won't even notice it.
iMacros is great for any repetitive tasks
User Agent Switcher is fun for people who check which type of browsers people use to access their websites.
YSlow is great for working out why a page loads slowly
StumbleUpon as others have mentionned
Firebug is the awesome must-have for anyone developing websites