I use these extensions for Firefox:

• Adblock Plus
  Ads were yesterday!
• Add Bookmark Here
  Add a menuitem "Add Bookmark Here..." to Bookmarks Menu.
• Bookmark Favicon Changer
  Right click your bookmark to change favicon.
• CheckPlaces
  Checks bookmarks are valid
• DivX Plus Web Player HTML5
  Increase performance and video formats for your HTML5
• DOM Inspector
  Inspects the structure and properties of a window and its contents.
• Download Statusbar
  View and manage downloads from a tidy statusbar
• DownThemAll!
  The mass downloader for Firefox.
• Element Hiding Helper for Adblock Plus
  Helps you create element hiding rules for Adblock Plus to fight the text ads.
• Extension List Dumper
  Dumps a list of the installed extensions.
• feedly
  Feed your mind. Anytime. Anywhere.
• Firebug
  Web Development Evolved.
• FireGestures
  Executes various commands with mouse gestures.
• Greasemonkey
  A User Script Manager for Firefox
• LastPass
  Last Password you will ever need
• Open Bookmarks in New Tab
  Always opens new tab from bookmarks.
• QuickDrag
  Extends the function of the drag-and-drop mouse gesture so that it can be used to load URLs, do a web search of selected text on a page, or save an image on a page.
• Rainbow
  Color tools for web development
• Reddit Enhancement Suite
  A suite of tools to enhance Reddit
• Selenium IDE
  Record, edit and play Selenium tests
• Stylish
  Restyle the web with Stylish, a user styles manager.
• Tab Badge
  Adds a counter badge to your tabs great for Facebook or Twitter notifications.
• Web Developer
  Adds a menu and a toolbar with various web developer tools.
• Xmarks
  Bookmark Sync and Web Discovery