If you do any kind of development for the web, then you know how important tools are, and you like finding tools that make your life easier. Developing and testing new browser features, however, takes time. Between the time a useful tool first appears in an experimental nightly build and the time it’s available for everyone to use in Firefox, a while has passed.
That’s one of the reasons Mozilla released Firefox Developer Edition in November 2014 as the recommended Firefox browser for developers. It gets new feature updates more quickly so that you can use the latest tools.
Source: Smashing Magazine