Excellent add-on, I recommend it to anyone trying to reclaim control over their time and attention.

I love how it's fully customizable, including:
- particular times of the day when a certain group of pages should be blocked
- how long each of them can be viewed (per day / hour / whatever you like)
- whether or not a site should be hidden from sight only when loading, or also while reading passively

It also displays a tiny timer indicating how much time you have left for today on Facebook or other distractions ;)

This extension is a nice improvement from the old (non-NG) version of the plugin, which already beat the competition hands down.