Unfortunately not working as it's supposed to. First of all, I had to enable mixed-content to get it to work at all via setting "security.mixed_content.block_active_content" to "false" in about:config.
After that I had to click on an image or a video that is innately on a games store page, only after that I got the YouTube gameplay-videos to play. Even after these steps though I experienced the gameplay-videos getting overlapped by the stores' game pictures, which automatically scroll through.
Hope there'll be an update in the future. Really wanted it to work, but the extension is not practical in its current condition.
I cant tell you how many times I pissed and moaned, about going to you-tube and looking up the game video game-play wile in Steam app OS. In-beaded feed is a You-Tube video... So thank you so very much author'(s) for you time and work. It really is a luxury app, But only makes total sense.