The Mozilla Foundation has an alternative to its stable client where they publish test versions of its browser that will be officially distributed in the future. Firefox Developer Edition has now replaced the Aurora channel to offer an updated version of the browser each month and a half, long before its launch on the standard channel, and also has some extra functionalities focused on web development.
This version includes several tools to create, edit, and test projects on several devices and browsers, with all the new features grouped into a drop-down menu in the toolbar. It includes the WebID system, a development kit to create webpages and applications for Firefox OS, and also has an extension to view and edit projects on other devices like smartphones and tablets, and another to check pages on other browsers like Chrome or Safari.
Its other features include options to inspect the page's source code and modify the HTML and CSS directly from the browser while viewing the changes in real time. It also offers debugging and performance-analysis options that allow, for example, checking the status of the network when sending online requests, or viewing a change log with all the interactions you do to a page.
For the standard user, the only difference from the original Firefox is a darker theme and the aforementioned features that aren't yet included on the stable channel, making this version ideal for developers as well as those who want to be privy to the latest developments on their favorite browser.