The addon is a webextension version of the Leechblock addon and helps me not wasting time (apart from writing a review when I should be studying ;-)).
You can block a list of domains or load a list of domains from an url. You can configure the day(s) of the week and the hours and minutes (eg. 0700-1700).
You can block yourself from undoing these settings and even disable the addon page of Firefox (be careful, you could lock it forever) to hinder yourself from disabling the addon.