Great thing, but I was already using "Youtube enhancer" to block ads. I use this, ABP, to stop the annoying video thumbnail previews on youtube, and for no other reason. It stops this annoying feature: the videos used to start playing a 3 second preview when you hovered the mouse on the thumbnail, but not anymore. BUT: websites are aware of this extension (even if you turn it off for that website) and they can annoy you. Youtube seems to detect the extension, and force all my videos to HD 1080, or whatever is the highest setting. That chews through my data too quickly.

Also: Other websites detect you have this extension, and tell you how to deactivate it (whitelisting) for their website only. They know you have the extension, but they DON'T know whether you have turned it off, so you can ignore their instruction and just refresh the page.