If I click the Play button, it just instantly pauses itself again.
If I hold "K", the video plays at a little over 50% speed due to constantly switching between paused and unpaused. The pop-up bubble in the center always pops up with the Play symbol.
This problem only occurs sometimes. Reloading the page fixes the problem most of the time. But it's very annoying and I'd prefer to prevent it from happening.
My system info: Xubuntu 18.04 64-bit, Firefox 61.0.1 64-bit, GTX 770 GPU.