No, this is impossible. Sorry. :(
Silverlight is proprietary; run by Microsoft, if I'm not mistaken. As for the sites that don't work with Moonlight, well, you're screwed unless you can get Virtualbox to work for you.
Moonlight works with Firefox these days (I'm running 19.0 right now, and it's working,) but, so far, I have never, ever, seen a single thing that supported Moonlight: video, service, anything.
If you've installed Moonlight on your Firefox after updating, but it still appears not to work, I'd recommend contacting the site's feedback centre to let them know how much you'd appreciate Moonlight support.