Is it just me, or it is a bit hypocritical for Mozilla to be talking about the wonders of the Open Web and then embracing two very proprietary developer platforms to promote Asm.jsThere is no contradiction here, on the contrary (heh!). Mozilla doesn't believe an Open Web precludes proprietary applications. In fact they've strongly argued that the web should be properly standardized so that it's possible for anyone, including commercial parties, to make clients for it, and that just opening up source code (ala WebKit, PNaCl, etc) does not do this hence is a bad way to achieve an open web.
The point has also already been made that this makes it possible for Unity games to run on open platforms without requiring closed source blobs.
they would be helping open source game engines
Uh, emscripten and asm.js have been enhanced to properly port many open source engines and game libraries long before this one was announced. I'm going to guess Unity was possible because they provided engineering resources to port their own engine.