As a free software enthusiast, I have mixed feelings about this.
On one hand, it's Mozilla that we know and love, it's free software and it's an effort driven by a community project and run by that.
On the other hand, the platform not only seems to accept proprietary components (like rild) as a compromise, but even embraces them and tries to actively support closed source components (the bug report about the libc counts the fact that uClibc & (e)glibc are GPL'ed as a disadvantage!). Additionally, it speaks about an "open platform", as in the Web, with open standards and all that, which of course says nothing about the freeness of such (Web) applications or reassures me about preserving my essential freedoms. Not unlike Android, but a big difference with free operating systems and platforms that most people around LWN are accustomed to.
All in all, it seems to me as an effort to build a proprietary platform with a free software layer between closed-source components and well-defined APIs to interface between them. What's the point? We already have Android for that, what we really need is a *more free* platform!