The whole point is that doing it in JS avoids the need for a native code plug-in, which cuts off an entire avenue of security problems. Same story as PDF.js.
I don't mean to imply that this project isn't impressive -- it is. But I hate to see them duplicate so much effort for a solution that is, by nature, performantly disadvantaged. We need a serious challenger to Flash Player (we've had enough toys) and Mozilla is in a good position to provide that, so I hope they choose to do so.
> You don't think they can get the code portable enough to work on multiple platforms?
Not on platforms that expressly disallow it, e.g. iOS.
>They don't include animations, they don't accept user input, they don't connect to servers and stream files and send updates
A PDF file is much more than just words and pictures. There are forms which accept user input, weird embedded content, and other strange things. Although I am not familiar with the spec, I have seen the features that Adobe Acrobat lets you do with PDFs and it seems pretty monstrous.
To clarify, there are several security issues that arise from all of this , and as others on this page have pointed out, that is a good reason to make use of the existing infrastructure around browser sandboxing.
 - http://duckduckgo.com/?q=adobe+pdf+vulnerabilities