Oh, man, I remember when I had this add-on. It was almost life-saving. (If I were a more dramatic person, I would've said it was DEFINITELY life-saving.) I think I stopped having it after numerous Firefox update that made the version I used incompatible. I haven't felt the need to block certain sites again, thanks to the disciplined habit this add-on helped me build. But now, I'm slipping on the procrastination track, and no better time than to re-install this add-on.

The add-on is highly versatile and doesn't dictate you on how and when to use it. Multiple options make it highly customisable to your own needs, including multiple options of site-blocking groups (i.e., you don't have to worry about needing to access one site while it's being blocked together with another site -- put them in different group, activate them separately, and you're good), scheduled blocking time, ability to start blocking at any time you want, and the option to override the block if you desperately need to (personally not recommended to use unless it's a life or death situation).

If you're looking for a site blocker that's easly altered to your needs as well as easy to use, this is the one for you.