It does exactly what it says in the description of this extension. It's the best Rick Roll blocking extension I've found on Firefox so far!

Just a heads up, this extension works by checking the link of the website you are currently on, so if you come across a rick roll video inside another website it won't block it (you have to click the link). Most (if not all) other rick roll blocking extensions also work like this, however. There are some things that could be added to this extension, such as a way to import/export your links as a .txt file so you can download/share your links with your friends so they can start using the extension quickly (it takes a while to get a decent selection of links to be blocked). Also, adding links can be a bit of a painful task because if you just select to add the current URL of the video to the list, then if you find the video in a playlist (URLs for videos on YouTube are different if it's a playlist), the video won't be blocked. This can be fixed by adding an * symbol to the end of the link, but it would be easier if you didn't need to do this.

Despite this, the extension works perfectly.