Stop addictive sites from stealing your focus by setting up a countdown timer.
The tab is destroyed when the countdown reaches 0. Default countdown is 3 minutes but you can adjust that for each site. You are notified when 30 seconds remain. To stop the countdown, click the stopwatch icon in the address bar.
Get this extension on Firefox: https://addons.mozilla.org/en-US/firefox/addon/exploding-tabs/