Only works sometimes. Seems to work with new YouTube pages opened.
However, I have pinned You Tube pages to Tabs bar at the top of my latest Firefox (61.0) 64-bit browser, so I can keep watching the long video the next day. When I restart my PC and browser the YouTube video starts to play again automatically, even though that pinned Tab isn't the active visible Tab. This is disappointing if you have to leave the PC after restarting, as that is the main reason I installed this app.

Add-on works more than half of the time. Here is the latest YouTube link that did not work: https://www.youtube.com/watch?v=qTNHQRKQ-e8