The tooling in Firefox does seem to be improving rapidly (kudos to the devs for that, I'm not trying to trivialise the hard work that they've been putting in, by any means), but there are still several basic(?) features missing from the script debugger. Calling this a "developer edition" is IMO a misnomer until you can reasonably use it to develop pages/sites/applications - currently every other major "not-for-developer edition" browser already gives you almost everything this does, and in some areas quite a bit more.
What would make it a developer browser to me:
* Folder grouping on resources
* Allowing webide or the script web tools tab to work with local folders (Chrome workspace equivalent)
* Dynamic updates to scripts (Chrome workspace/dev tools equivalent)
* The ability to open and/or display more than 1 script at a time. Tabs in developer tools should operate like browser tabs (orderable, poppable etc)
I hope that this isn't just a re-branding exercise - the video, site and fanfare make it sound like Mozilla's aiming to make something great for developers (not to mention that the FF tools are headed in the right direction), but the first release and associated posts/comments seem to indicate that it's essentially a nicer packaging of what used to be aurora.