[godawful]

Try Leech Block:

https://addons.mozilla.org/en-US/firefox/addon/4476/

It blocks selected websites during hours / days you select, or allows you to say "No more than 30 minutes of Hacker News every 4 hours." It's highly configurable.

Of course, you're smart enough to find a way round its blocking if you want to. But really, deep down, you want to be productive and when Leech block pops up with its "Site blocked" screen, it will be a helpful reminder of this.