The Mozilla Update site has lots and lots of extensions for Mozilla browsers. Here are my picks:

• Autocopy - automatically copy any selected text to clipboard
• Tabbrowser Preferences - tweak tab settings (includes putting an X on each tab label, not just on the right)
• AdBlock
• ForecastFox - weather forecasts in your toolbar
• DictionarySearch - get definitions via context menu
• Web Developer Toolbar - highly recommended if you develop websites
• ConQuery - query web services from the context menu; good for fiddling around with server-side scripts
• GooglePreview - adds thumbnails of result webpages to Google and Yahoo search results
• Javascript Debugger - aka Venkman
• Nuke Anything - probably superceded by AdBlock, but more straightforward. And perhaps you like advertisements…
• EditCSS - modify stylesheets, live
• AutoForm - automatically fills data into forms, such as your name and email address.

Potentially interesting:

• ieview - preview a page or link in MSIE
• SmoothWheel - smooth scrolling when using mouse wheel
• QuickNote - make annotations on web pages

I've tried a bunch of these and they work as advertised. It's great that Mozilla's extension system makes it possible for so many people to improve the product.