Mozilla has a website add-on section that has thousands of useful add-ons in different categories. Some of these add-ons are useful for penetration testers and security analysts. These penetration testing add-ons helps in performing different kinds of attacks, and modify request headers direct from the browser.
This article lists a few popular and interesting Firefox add-ons that are useful for penetration testers. These add-ons vary from information gathering tools to attacking tools.