BlockClock (also known as Strict Workflow) by lolrepeatlol

This is an extension that uses the "pomodoro" technique. It's strict, and doesn't let the user cancel a timer. It also blocks websites.

This extension is a fork of the unmaintained Strict Workflow extension, seen here https://github.com/matchu/Strict-Workflow. Right now, this doesn't have much more than being Firefox compatible with a nice rebrand. More features will be added with time.

This extension is open source: https://github.com/lolrepeatlol/BlockClock