The Firefox Developer Edition is a special version of Firefox that has been tailor-made for developers. It features all of the latest Firefox features along with a number of specialist developer tools.
After installing the Firefox Developer Edition, you’ll notice that it looks slightly different to the standard Firefox toolbar. Firefox has certainly taken a more developer-centric approach in designing its toolbar, making it narrower in appearance and decorated with far more buttons by default.
The default theme for Firefox Developer Tools is dark, which is likely to be the result of user testing. However if you don’t like the dark theme, you can always switch the developer edition theme off by going to Menu > Customize.