I've used Leechblock (firefox extension) in the past for the same purpose. One feature it has which I found particularly useful is the option to delay pages ins

I've used Leechblock (firefox extension) in the past for the same purpose. One feature it has which I found particularly useful is the option to delay pages instead of completely blocking them. Instead of the transition being an abrupt stop that makes me want to turn the blocking off to finish what I was doing, the delay makes it a gradual frustration, and feels a lot more like getting naturally bored of browsing.