I declare war to rouded corners!

* {
  border-radius: 0 !important;
}

This CSS tells your browser that everything (*) should have the property border-radius (amount of rounded corner) to 0.

The extension just injects this into your page after checking if the page is whitelisted. This extension is so tiny that you can review mango.js and main.js by yourself or with help of AI.

Mango is the codename of Windows Phone 7.5, the first version to adopt Microslop's "Metro UI". I grew up as a kid using Windows Phone and that shaped the way I see rounded corners—and I absolutely hate them.