Firebug (https://getfirebug.com/) is a Firefox extension commonly used by developers to detect errors during the execution of web applications. But it can also be used to assess applications and understand abnormal behaviors. Currently, Firebug is included in Firefox's Developer edition by default. To access it, you just need to do a right-click on a website and select Inspect Element, as shown in the following screenshot:

The browser will show the frontend code and analyze it:

Firebug is very useful in exploiting vulnerabilities, such as cross-site scripting (XSS), template injection, and SQL injection. It is a perfect tool to create testing pages for XSS, where a copy of real content to spoof a website is needed.