The extension was adding a bunch of features to make the gameplay more enjoyable. Most of the modifications were happening at the UI level, but it also provided integrations with third-party services. It was one of the most popular extensions for the game at the time, and was regularly recommended.
The extension was developed with Vanilla JS, with a handmade layer of code to allow cross-browser compatibility.
I stopped working on the script when I stopped playing back in 2014. But the project was forked and is still actively maintained.