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• Nuke Anything – probably superceded by AdBlock, but more straightforward.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.