I use leechblock. It's pretty great. It's extremely feature rich. The UI is a little ugly but "ugly" in the style that a hackernewser would likely appreciate.
My only complaint is the way it does this thing when you're out of time and it redirects you (either to their block page or a custom page you set up). Not sure how it does it but the original url you tried to reach doesn't get saved in your history. If I try to manually disable the extension it'll also automatically close the page with the leechblock message so it's gone forever. Kind of annoying if it's a link I'd like to revisit after work
