LeechBlock NG – Get this Extension for 🦊 Firefox (en-US)
addons.mozilla.org
Download LeechBlock NG for Firefox. LeechBlock NG is a simple productivity tool designed to block those time-wasting sites that can suck the life out of your working day. All you need to do is specify which sites to block and when to block them.

Allows setting time limits on sites. To stop yourself from wasting your life on dumb stuff.