Excellent Add-on Puntuado con 5 de 5 estrellas
This is a superb Add-on, I use it to block several sites.
To dr3adlok in the user review below, you are mistaken, there is a user interface for automatically blocking web sties. Just right click anywhere on the page of the site you want to block, and choose the option (Block This Site), then the site is automatically blocked for you.
P.S. I'm using version 1.0.19