Flash, Java, Javacript, et al are great for the web because they are "write once, run anywhere". The same source code runs everywhere. That is what the web is all about. HTML and CSS are not scripting languages and they are also "write once, run anywhere". That is how the web delivers your programs.
Now, what will your Native Client do?
It will have COMPILED code for ONE platform. Like in C++ where you "write once, compile everywhere". Except you probably won't be able to compile everywhere. The point of the web is that any platform should be able to run your app.
Look, I agree that it's more of a philosophical thing, and indeed you are welcome to make an extension for Mozilla and all the other browsers. But the security risks alone will make this a tough sell to INCLUDE in a browser -- harming the spirit of the web. Not only that, but the web is totally against "favoring one platform over another" ... it is BECAUSE of the web that the platforms are able to work better together.