The true delight is in the finding out rather than in the knowing. — Isaac Asimov

The must for Web developpers:

• FireBug
• YSlow
• WebDeveloper
• Selenium
• Aardvark
• Html Validator
• MeasureIt
• ColorZilla

For daily use:

• Foxmarks
• Download Statusbar
• Fasterfox
• Modify Headers (user-agent -> Googlebot/2.1 (http://www.googlebot.com/bot.html) )
• Advanced Bookmark Search
• MID (Internet Dictionary)
• All-in-One Gestures