Accoridng to the Netflix support pages (and I tested and confirmed it on my computer), in order for Firefox to be able to play Netflix video, you need to:
- Enable playback of HTML5 DRM media in Firefox.
- Try to watch the DRM-protected Netflix video again.
- Wait for Firefox to automatically install the "Widevine Content Decryption Module provided by Google, Inc" plugin (Firefox will automatically install it when you first try to watch any Netflix video).