There are good and bad things about PNaCl. But overall, the resistance to PNaCl goes beyond Mozilla -- it's a really hard standardization/synchronization problem. Our goal with asm.js was to produce something that could work today (because it's 100% compatible with existing JS semantics) and work even better tomorrow as engines optimize it even better.

Edit: Re: predictable performance, that's exactly what asm.js is about. The performance model is much closer to that of C or PNaCl/LLVM.