Firefox Developer Edition is a version of Mozilla product created specifically for software developers. It has several new features and unique functionality. This version of the browser can actually operate independently of standard Firefox and family browsers like Safari. It borrows the default colors and interface elements from the developer tool set which makes the interface itself easy to use and navigate if you have any experience in development of products in this line.
Some of the new tools, like the Valence add-on, give you access to Firefox developer tools for other browsers. Other notable features include things like Responsive Design View, which lets users change screen sizes in real-time without adjusting size of their browser window. Then, there's the style editor, a tool that lets you edit CSS page styles, to create your own variations and apply existing CSS style sheets to other pages.
Firefox Developer Edition is an all-in-one toolbox for people into browser development. It has all the basic tools and some of the more advanced features one would expect from a software bundle like this.