Very cool that they managed to get SpiderMonkey working through Emscripten. But I wonder if they could have just used Narcissus[1] instead and saved themselves some time.
Narcissus only runs on spidermonkey because it uses nonstandard spidermonkey extensions to work. Unless I'm misunderstanding the implications of the first paragraph in the link you provided.
The very awesome twitter demo on the js.js site is one example of something you could no way do with narcissus except as a proof of concept in FF.
[1] http://en.wikipedia.org/wiki/Narcissus_%28JavaScript_engine%...