For web development

• Colorful tabs -- Colors each tab differently and makes them easy to distinguish.
• ColorZilla -- Eyedropper
• Firebug -- Prereq for ySlow
• YSlow -- Reports on page download time with optimization tips
• IE Tab -- Renders pages as IE would
• Web Developer -- Classic
• Selenium IDE -- Record and playback web site testing utility

Other

• Sage -- RSS and Atom feed aggregator
• TableTools -- Sort, filter or copy tables
• Distrust -- Hide surfing trail withing browser