Nobody mentioned the Leechblock Firefox plugin yet. Of course it only helps you by blocking chosen sites after a predefined spent time (highly configurable), it

Nobody mentioned the Leechblock Firefox plugin yet. Of course it only helps you by blocking chosen sites after a predefined spent time (highly configurable), it won't solve your problem for you. But at least it gives you the "trigger" by bringing your attention to the fact you've already spent your alotted time goofing off.