Pomodoro Timer от P0m0d0r0Guy

The Pomodoro Timer add-on for Firefox helps you work efficiently using the Pomodoro principle. It features a timer that alternates between 25-minute work periods and 5-minute break periods, with notifications and sound alerts to keep you on track. This project is totally open-source : https://github.com/P0m0d0r0Guy/pomodoro-add-on