Just a quick note about my most favourite add-ons used in FireFox,

• Copy Plain Text
  Copies text without formatting.
• Delicious Bookmarks
  Access your bookmarks wherever you go and keep them organized no matter how many you have.
• Fasterfox
  Performance and network tweaks for Firefox.
• Firebug
  Web development evolved.
• FoxyProxy
  Premier proxy management for Firefox.
• IE Tab
  Enables you to use the embeded IE engine within Mozilla/Firefox.
• Live HTTP headers
  View HTTP headers of a page and while browsing.
• Live PageRank
  Display live PageRank for active URL.
• PDF Download
  Allows you to choose what to do with a PDF file: download it, view it with an external viewer or view it as HTML.
• Tab Mix Plus
  Tab browsing with an added boost.
• Web Developer
  Adds a menu and a toolbar with various web developer tools.