hello zuzann, first of all please urgently update firefox and all your existing plugins - there were dozens of highly critical security vulnerabilities fixed since firefox version 5!

Update Firefox to the latest release

https://www.mozilla.org/plugincheck/

& regarding silverlight: you'd have to install the 32-bit version, since firefox is also a 32-bit application at the moment and won't work with plugins that are 64bit.

I had the same problem but found the fix!

https://support.mozilla.org/en-US/questions/935535#answer-383515

They said to 'reset' firefox. I followed the instructions and now I can watch Netflix with no problem.