For web security researchers and anonymous usage. Here are some useful add-ons. Not everything you need to download just keep what handy for you. More add-ons here

• Modify Headers
• Cookies Manager+
• Disable WebRTC
• HackBar
• HTTPS Everywhere
• PassiveRecon
• SQL Inject Me
• XSS Me
• Tamper Data
• User Agent Switcher
• FoxyProxy
• Live HTTP Headers
• AdBlock Plus
• No Script
• FireBug
• Web Developer
• Wappalyzer