I’m a developer, so the most are web development extensions, but I didn’t forget others…
These are my favourite add-ons for Firefox 3:

• Colourful Tabs: it colors your tabs, your browser just looks nicer
• ColorZilla: advanded colorpicker
• Delicious Bookmarks: easy and rapid way to embed your bookmarks with Firefox and sharing them
• Download Statusbar: shows your downloads in a comfortable toolbar
• Firebug: The developer extension, simply complete of all tools ad essential
• Gmail Notifier: be immediately notified on mail received on your GMail box
• GoogleToolbar: I use it for the very useful translator
• GooglePreview: shows thumbnails preview for every page found in Google searches
• Html Validator: w3c html code validation check
• Hyperwords: selecting a word or a phrase it adds a lot of operation with: search, translate, post to blog…
• IE Tab: test your site with Internet Explorer into Firefox
• JSView: lists all Javascript and Css files of the page and open them with your favourite editor
• McAfee SiteAdvisor: a security tool which tell you if a site is trustable
• MeasureIt: extensions useful to measure what you want in the page
• PdfDownload: prompt you every time what you want to do with a .pdf file
• StumbleUpon: discovery new sites based on your interests and share it with friends
• Yahoo! Mail Notifier: be immediately notified on mail received on your Yahoo! Mail box

And what are yours?