Pomodoro clock extension โดย macmorning

Use this extension to organize your work using the well-known pomodoro timer technique: work X minutes, have an Y minutes break, and so on. You can configure the duration of each streak as it suits your own pace. The extension will display a notification and play a short ring at the end of each timer.

The source code is available on Github along with a few explanations.
Please don't use reviews to submit your issues :) You can post them on Github, like enhancement requests. All constructive comments are welcome.