Over the last few days NOW TV users with Avast Antivirus have been unable to watch on their PCs or Macs via their browsers due to a "Silverlight" issue.
The fault is that Avast antivirus is wrongly detecting a file, which then blocks NOW TV from running.
If you see the Threat blocked warning or use Avast and get the "A Silverlight error has occured" and it's worked fine before open Avast.
Click on the Cog at towards the top right
Scroll down until you reach Exclusions
Click on the Urls tab
And enter *.nowtv.com/* and click Add
You should now be fine to watch NOW TV again.
Note I've just done the whole setup from start to finish installing Avast, running Firefox, getting the error and finally being able to watch Sky Movies Oscars, so it should work for you.