The inner sandbox is, at the moment, far less trustworthy than the outer one. That's because it encapsulates content in an iframe to not break out and access the iframe's parent document. We'll change this in bug 1137942, after which we should be able to make much stronger statements about this sandbox. That'll still leave the communication protocol, which is somewhat messy right now. We'll clean that up in bug 1137949. Not requesting review of this part for now.