History

• Before the 1990s, most web pages were static, displaying text and images with HTML (Hyper Text Markup Language) and CSS (Cascading Style Sheet). Javascript became ubiquitous in the late 1990s and, with the advent of Web 2.0 technologies, makes up a huge portion of the software development demand for web applications. Firefox became one of the most versatile environments for Javascript debugging with its rich native developer tools and add-ons that allow comprehensive testing and examination of the Javascript run-time environment.
  
  

Native Features

• The "View Source" and "Error Console" features in Firefox allow developers to inspect statically written code for a webpage. These tools show the big picture of what is going on "under the hood," reporting detailed information about any Javascript warnings or errors encountered at which positions within the code. To ease understanding, Firefox even provides syntax highlighting when you are viewing source code.
  
  

Firebug Add-On

• For dynamically generated Javascript code, Firefox native features fall short due to inherent limitations in how these are implemented. Firebug is a popular web debugging tool that allows programmers to view both dynamically and statically delivered code, set breakpoints and control execution of Javascript, examine AJAX requests and even peer into the DOM (Document Object Model) of the page interactively. Developers can even edit the Javascript code in real time, which provides for easier testing and debugging of new features.
  
  

Web Developer Add-On

• The Web Developer Add-On for Firefox offers functionality much like that in Firebug, but it provides more granular control over how Javascript interacts with the static elements of webpages. Web Developer enables developers to fine-tune how the website interacts with users in real time while debugging. Web application testers use this tool to manipulate static elements on the web page to verify and enhance Javascript functionality.
  
  

Limitations

• The main limitation in debugging Javascript in Firefox is the lack of a cross-platform Javascript standard. Since different browsers implement the Javascript interpreter in different ways, a process that debugs correctly in Firefox may not necessarily work in Internet Explorer, Safari or Chrome. Although development tools in Firefox provide advanced debugging features for Javascript, they cannot eliminate cross-platform errors inherent to proprietary browser implementations.