Why a different browser and not just an extension? I feel like different browsers leads to different versions of engines, languages etc. aka a lot of headaches. As a web developer, I want to see what my users see, not what "developers" see..
But other than that, it's basically Firefox.
This is different.
It's not revolutionary (we're not releasing Servo yet), but it is pretty fantastic. I'd be shocked if most Firefox Aurora users don't switch over to the developer browser next week. :)
Is it? Sounds like a side-by-side Aurora with a toolbar button for DevTools. No separate feature set AFAICT, but you tell me if I'm wrong.
I think it's great that Mozilla is exploring new territories; it may or may not turn out to be a good idea, but give them the benefit of the doubt, at the end of the day no matter how good web developers tools currently are, without experimentation there would eventually be stagnation.
But my 2c, is that even if the project does not work out as hoped, what ever they are doing can put turned into ad add-on.