Some Firefox users noticed playback issues on YouTube for several months. These affected high resolution videos only, from 1080p and up. To make matters worse, no clear pattern could be identified.
Some videos played fine, others would stop abruptly when they ran out of buffer.
The DRM module tells Netflix you are on Firefox. For Netflix I would use Chrome on Wine tbh, it now supports 1080p w/o extensions. Or maybe Chromium w/ ChromeOS’ Widevine module + CrOS user agent.