OutOfYoutube автор LightStack

This Add-on makes it possible to keep your usage of YouTube in a selected time frame. Every time you try to watch a video outside of chosen time window, you get redirected to a predefined YouTube video (or just link).
You can change the time frame as well as the redirect URL in the ad-don's preferences.

I created this Extension, so I won't watch videos until 2 in the morning anymore, but it fits a lot more purposes.
I hope it somehow improves your browsing experience,
LightStack