As I'm sure you already know, currently, out of the box, it features
WebIDE allows us to develop, deploy and debug web apps. Video
Responsive Design View - see how the website will look in different screen sizes
Valence (Debug any browser) - previous called Firefox Tools Adapter) . Video
Web audio Editor - inspect web audio to ensure all audio nodes are connected as expected.
Page inspector - examine HTML and CSS
Network monitor - see all network requests the browser makes and how long it takes
Style editor - edit CSS styles
See a general video introduction
Further, it also includes
- Firefox Hello which is a WebRTC (a tool allowing call and video chat with others from within the browser).
- Forget button which is similar to clear history.
- Eyedropper (to 'snap' colours from the webpage)
- Connect - connect to a remote device
As you may notice, some (most) of these are already in Firefox or Firefox plugins today... And as time goes on, I suspect will be available in plugins tomorrow. For example, the Page inspector, console, debugger (and a few more) are already in Firebug.
As for changing the screen sizes and CSS, I used a tool called Developer Toolbar...
I have used it as a web developer and designer and it just feels like Firefox with plugins designed to suit a web designer (which is what I think they were aiming for). I'm personally more interested to see how it develops.
At the moment, it feels very similar to the debugger already built into Chrome and IE11 (although I have not played with every feature)
You can also view the FireFox Developer Edition Notes for more detail