If NativeClient is available as a plugin, can't Firefox users benefit from the technology regardless of whatever Mozilla thinks? I like the idea of lightweight x86 sandboxes like Native Client and vx32, but I understand why Mozilla isn't very interested in them. As long as nothing is done to actively hinder their development and use by those who are interested, is there really a problem here?
The problem with plug-ins is that relying on them causes a huge drop-off in usage, at least in my experience. Requiring a download of some kind, even if it's just a plug-in, costs you about 30% of your possible users.