Block websites for a given time to help you focus on work/study. Gives you more flexibility then traditional schedule based blockers
FocusBlocker is a light-weight add-on that blocks time-wasting websites to help you focus on real work. 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.
Firefox version: https://addons.mozilla.org/en-US/firefox/addon/focusblocker-new/
How It Works
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1. Set a timer, say 10 min.
2. Browse freely for 10 min.
3. When time is up, the blacklisted sites will be blocked for 50 min (configurable).