• Adblock Plus
• BugMeNot (bypass free registration)
• ChatZilla (IRC client)
• Greasemonkey (everything)
  • YouTube Prevent Autoplay
  • YouTube Automatic Fmt=18 Adder
  • Download YouTube Video III
  • YouTube "Lights Out"
  • Nested Twitter Replies
• Lazarus (saves form text)
• LJLogin (for multiple LJ accounts)
• Mouse Gestures Redux (would be better if right button of mouse hadn't broken)
• PDF Download
• Popup ALT Attribute (pops up alt text when hovering)
• Read It Later (saves pages to read later)
• Screengrab (saves a page as an image)
• Tab Mix Plus
• Undo Closed Tabs Button
• Zotero (great for academic research)

Disabled

• Delicious Bookmarks (neat, but inconvenient when also using Firefox bookmarks)
• Livejournal Addons
• Tab URL Copier (never used)