I'm wondering this as well - why can it not be implemented (from the browser per...

I'm wondering this as well - why can it not be implemented (from the browser perspective) as a plugin that is downloaded upon first use?

Isn't this what Iceweasel on Debian will have to do anyway, since Debian cannot ship non-free software in their main repositories?

Yes, it will be a plugin that is downloaded on first use. That's exactly what the blog post describes.