Boot To Gecko will incorporate new yet-to-be-developed web APIs, provide a privilege security model, booting to Android-compatible devices, and applications can choose to port or build applications. The new web APIs will expose new device capabilities as well as that of the operating system, including telephony, SMS, camera, USB, and Bluetooth, among other areas.
The B2G / Boot To Gecko work is currently documented on the Mozilla Wiki and this mailing list thread.
Source code to this Mozilla project is being released in real-time via GitHub.
From the discussions thus far, it appears Mozilla Boot To Gecko is looking towards a Google Android base for its compatibility in the mobile device space where they're looking for B2G to be most utilized. The developers also aren't interested in the ability to run native code on the device.