YouTube videos in Firefox show this weird overlay which forces me to watch it in another tab.

(I'm reposting this AGAIN because my photos were too small in the other one.)

So I click on it and it plays in the other tab. But the original window and the familiar red YouTube arrow are GRAYED OUT and unusable. I don't know what's going on, but it only does this in Firefox. I'm attaching two screenshots, one showing the original YouTube video page, and the other tab page in which I play the video. Since when does YouTube make you open another tab to play a video? It should play right IN the same window at the page I selected!