Hey all. I might have something on this. I trawled this board in frustration a few weeks ago and tried all the answers shown and nothing worked. Interestingly, I did manage to get it going on internet explorer at that time but not at all on firefox. Anyway, I've just joined netflix and imagine my horror when I got a similar message. However Netflix told me how to fix it and their fix worked.
I went to Tools and then Options on the Firefox browser. In the Privacy tab I always have this set to "Never remember my history". If you change this to "Remember History" you then get an option to UNtick a box on something called Private Browsing Mode. Do this. Firefox will want to restart in order to operate the new settings. Once firefox has restarted you should be able to use Sky go. It worked for me on Netflix and I thought hang on a sec, now I'll try Sky go and that was working too.
Basically, it seems that if you have your firefox browser set to "Never remember my history" this means that your settings automatically enable Private Browsing Mode which in turn will not then play Netflix (and seemingly sky go?) through silverlight.
Remember though - now you'll have to clear your own browsing history.
I hope this helps you all, I hope it wasn't just me.