In order to watch netflix with firefox under Ubuntu 18.04, you need to make sure that you have DRM enabled in your firefox settings (Go to settings, scroll down, check play drm-controlled content). Also make sure you have the libavcodec-extra package (from the universe repository) installed. In order to install libavcodec-extra you should have the universe repository enabled:

sudo add-apt-repository universe
Then install the libavcodec-extra package:

sudo apt install libavcodec-extra