FocusBlocker (New) by Shing Lyu

FocusBlocker is a light-weight add-on that blocks time-wasting websites to help you focus on real work. By clicking the icon, a selected list of websites will be blocked for a given time (say 30 min). Also, a soft reminder will pop-up when you browse those websites for too long. FocusBlocker can give you flexible control over when you want to block, which is superior than a schedule-based approach is you have a flexible working schedule.