Anyway, will try to keep it short; but basically this causes YouTube to load a black screen. I can see the video buffering, but nothing is playing.

I click on it and then it switches to PAUSE icon. I click on it again and then it does the spinning loading graphic. If I manually jump ahead in the video it plays normally.

I do not have FLASH installed. I finally removed this and just installed - DISABLE HTML5 AUTOPLAY - and everything works normally. No flash so Firefox should be loading HTML5 by default, but the autoplay seems to load a black screen. With this OTHER Add-On it is paused and the click loads and plays the HTML5 video fine.