Is that one of those things that "nobody" willingly installs in their browser unless they absolutely have to because much of the market doesn't even understand what they are and the ones that do still want to avoid them because they decrease browser stability?
Quite literally; there is a section of the Mozilla browser codebase that actively seeks out those plugins (along with QuickTime, Windows Media Player, and Acrobat Reader) because users who lack them experience a "broken web" (http://lxr.mozilla.org/mozilla-central/source/dom/plugins/ba...).
I'm not saying becoming one of those plugins is impossible---clearly, it's been done more than zero times. I'm saying that I really wouldn't base any business decisions around the assumption that it will happen for your plugin.
Just make the DOM available as some virtualized "system calls" and you'll avoid the fate of Java and Flash.