One of the best extensions you will ever install. Simple and easy to use, it's by far the best browser blocking add-on out there. Even just a 30 second time-out (you can chose the time) before letting you waste time on Twitter or wherever, is enough for you to dosconnect from mindless-browsing-brain and make a conscious choice about whether that is really what you need to do. Most of the time it isn't and you will close that tab immediately, but when you really do need to go to a time-suck site, you still can without too much interruption.