I have autoplay disabled in FF. The trick in Youtube is to double click the play button the first time. After that all works normally. (For some reason double clicking the video area doesn't produce the same result.)
I find a general difficulty with processing the meaning of "modal" buttons. If it's showing the "Pause" symbol, does it mean it's paused now, or that it will pause when I click it? I had the same problem with the stupid slider buttons that have become fashionable to use in place of checkboxes.