It doesn't work with the addon Firefox Multi-Account Containers, the player loads underneath the default Youtube player.

Doesn't work on 720. Might work on 480, but 480 streams fine for me.
It seems to increase the buffering speed overall of youtube, but will not complete an entire video. My video does buffer faster, but not fully.
Just tested if something might be blocking the extension:
about:config media.mediasource.enabled setting it to false will fully buffer a video that this extension only buffers around 20-40 seconds worth